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+--- Day 13: Packet Scanners ---
+
+You need to cross a vast firewall. The firewall consists of several
+layers, each with a security scanner that moves back and forth across
+the layer. To succeed, you must not be detected by a scanner.
+
+By studying the firewall briefly, you are able to record (in your
+puzzle input) the depth of each layer and the range of the scanning
+area for the scanner within it, written as depth: range. Each layer
+has a thickness of exactly 1. A layer at depth 0 begins immediately
+inside the firewall; a layer at depth 1 would start immediately after
+that.
+
+For example, suppose you've recorded the following:
+
+0: 3
+1: 2
+4: 4
+6: 4
+
+This means that there is a layer immediately inside the firewall (with
+range 3), a second layer immediately after that (with range 2), a
+third layer which begins at depth 4 (with range 4), and a fourth layer
+which begins at depth 6 (also with range 4). Visually, it might look
+like this:
+
+ 0   1   2   3   4   5   6
+[ ] [ ] ... ... [ ] ... [ ]
+[ ] [ ]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+Within each layer, a security scanner moves back and forth within its
+range. Each security scanner starts at the top and moves down until it
+reaches the bottom, then moves up until it reaches the top, and
+repeats. A security scanner takes one picosecond to move one step.
+Drawing scanners as S, the first few picoseconds look like this:
+
+
+Picosecond 0:
+ 0   1   2   3   4   5   6
+[S] [S] ... ... [S] ... [S]
+[ ] [ ]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+Picosecond 1:
+ 0   1   2   3   4   5   6
+[ ] [ ] ... ... [ ] ... [ ]
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+Picosecond 2:
+ 0   1   2   3   4   5   6
+[ ] [S] ... ... [ ] ... [ ]
+[ ] [ ]         [ ]     [ ]
+[S]             [S]     [S]
+                [ ]     [ ]
+
+Picosecond 3:
+ 0   1   2   3   4   5   6
+[ ] [ ] ... ... [ ] ... [ ]
+[S] [S]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [S]     [S]
+
+Your plan is to hitch a ride on a packet about to move through the
+firewall. The packet will travel along the top of each layer, and it
+moves at one layer per picosecond. Each picosecond, the packet moves
+one layer forward (its first move takes it into layer 0), and then the
+scanners move one step. If there is a scanner at the top of the layer
+as your packet enters it, you are caught. (If a scanner moves into the
+top of its layer while you are there, you are not caught: it doesn't
+have time to notice you before you leave.) If you were to do this in
+the configuration above, marking your current position with
+parentheses, your passage through the firewall would look like this:
+
+Initial state:
+ 0   1   2   3   4   5   6
+[S] [S] ... ... [S] ... [S]
+[ ] [ ]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+Picosecond 0:
+ 0   1   2   3   4   5   6
+(S) [S] ... ... [S] ... [S]
+[ ] [ ]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+( ) [ ] ... ... [ ] ... [ ]
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+
+Picosecond 1:
+ 0   1   2   3   4   5   6
+[ ] ( ) ... ... [ ] ... [ ]
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+[ ] (S) ... ... [ ] ... [ ]
+[ ] [ ]         [ ]     [ ]
+[S]             [S]     [S]
+                [ ]     [ ]
+
+
+Picosecond 2:
+ 0   1   2   3   4   5   6
+[ ] [S] (.) ... [ ] ... [ ]
+[ ] [ ]         [ ]     [ ]
+[S]             [S]     [S]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+[ ] [ ] (.) ... [ ] ... [ ]
+[S] [S]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [S]     [S]
+
+
+Picosecond 3:
+ 0   1   2   3   4   5   6
+[ ] [ ] ... (.) [ ] ... [ ]
+[S] [S]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [S]     [S]
+
+ 0   1   2   3   4   5   6
+[S] [S] ... (.) [ ] ... [ ]
+[ ] [ ]         [ ]     [ ]
+[ ]             [S]     [S]
+                [ ]     [ ]
+
+
+Picosecond 4:
+ 0   1   2   3   4   5   6
+[S] [S] ... ... ( ) ... [ ]
+[ ] [ ]         [ ]     [ ]
+[ ]             [S]     [S]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+[ ] [ ] ... ... ( ) ... [ ]
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+
+Picosecond 5:
+ 0   1   2   3   4   5   6
+[ ] [ ] ... ... [ ] (.) [ ]
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+[ ] [S] ... ... [S] (.) [S]
+[ ] [ ]         [ ]     [ ]
+[S]             [ ]     [ ]
+                [ ]     [ ]
+
+
+Picosecond 6:
+ 0   1   2   3   4   5   6
+[ ] [S] ... ... [S] ... (S)
+[ ] [ ]         [ ]     [ ]
+[S]             [ ]     [ ]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+[ ] [ ] ... ... [ ] ... ( )
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+In this situation, you are caught in layers 0 and 6, because your
+packet entered the layer when its scanner was at the top when you
+entered it. You are not caught in layer 1, since the scanner moved
+into the top of the layer once you were already there.
+
+The severity of getting caught on a layer is equal to its depth
+multiplied by its range. (Ignore layers in which you do not get
+caught.) The severity of the whole trip is the sum of these values. In
+the example above, the trip severity is 0*3 + 6*4 = 24.
+
+Given the details of the firewall you've recorded, if you leave
+immediately, what is the severity of your whole trip?
+
+Your puzzle answer was 1300.
+
+--- Part Two ---
+
+Now, you need to pass through the firewall without being caught -
+easier said than done.
+
+You can't control the speed of the packet, but you can delay it any
+number of picoseconds. For each picosecond you delay the packet before
+beginning your trip, all security scanners move one step. You're not
+in the firewall during this time; you don't enter layer 0 until you
+stop delaying the packet.
+
+In the example above, if you delay 10 picoseconds (picoseconds 0 - 9),
+you won't get caught:
+
+State after delaying:
+ 0   1   2   3   4   5   6
+[ ] [S] ... ... [ ] ... [ ]
+[ ] [ ]         [ ]     [ ]
+[S]             [S]     [S]
+                [ ]     [ ]
+
+Picosecond 10:
+ 0   1   2   3   4   5   6
+( ) [S] ... ... [ ] ... [ ]
+[ ] [ ]         [ ]     [ ]
+[S]             [S]     [S]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+( ) [ ] ... ... [ ] ... [ ]
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+
+Picosecond 11:
+ 0   1   2   3   4   5   6
+[ ] ( ) ... ... [ ] ... [ ]
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+[S] (S) ... ... [S] ... [S]
+[ ] [ ]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+
+Picosecond 12:
+ 0   1   2   3   4   5   6
+[S] [S] (.) ... [S] ... [S]
+[ ] [ ]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+[ ] [ ] (.) ... [ ] ... [ ]
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+
+Picosecond 13:
+ 0   1   2   3   4   5   6
+[ ] [ ] ... (.) [ ] ... [ ]
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+[ ] [S] ... (.) [ ] ... [ ]
+[ ] [ ]         [ ]     [ ]
+[S]             [S]     [S]
+                [ ]     [ ]
+
+
+Picosecond 14:
+ 0   1   2   3   4   5   6
+[ ] [S] ... ... ( ) ... [ ]
+[ ] [ ]         [ ]     [ ]
+[S]             [S]     [S]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+[ ] [ ] ... ... ( ) ... [ ]
+[S] [S]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [S]     [S]
+
+
+Picosecond 15:
+ 0   1   2   3   4   5   6
+[ ] [ ] ... ... [ ] (.) [ ]
+[S] [S]         [ ]     [ ]
+[ ]             [ ]     [ ]
+                [S]     [S]
+
+ 0   1   2   3   4   5   6
+[S] [S] ... ... [ ] (.) [ ]
+[ ] [ ]         [ ]     [ ]
+[ ]             [S]     [S]
+                [ ]     [ ]
+
+
+Picosecond 16:
+ 0   1   2   3   4   5   6
+[S] [S] ... ... [ ] ... ( )
+[ ] [ ]         [ ]     [ ]
+[ ]             [S]     [S]
+                [ ]     [ ]
+
+ 0   1   2   3   4   5   6
+[ ] [ ] ... ... [ ] ... ( )
+[S] [S]         [S]     [S]
+[ ]             [ ]     [ ]
+                [ ]     [ ]
+
+Because all smaller delays would get you caught, the fewest number of
+picoseconds you would need to delay to get through safely is 10.
+
+What is the fewest number of picoseconds that you need to delay the
+packet to pass through the firewall without being caught?
+
+Your puzzle answer was 3870382.
+
+Both parts of this puzzle are complete! They provide two gold stars: **