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+--- Day 1: Inverse Captcha ---
+
+The night before Christmas, one of Santa's Elves calls you in a panic.
+"The printer's broken! We can't print the Naughty or Nice List!" By
+the time you make it to sub-basement 17, there are only a few minutes
+until midnight. "We have a big problem," she says; "there must be
+almost fifty bugs in this system, but nothing else can print The List.
+Stand in this square, quick! There's no time to explain; if you can
+convince them to pay you in stars, you'll be able to--" She pulls a
+lever and the world goes blurry.
+
+When your eyes can focus again, everything seems a lot more pixelated
+than before. She must have sent you inside the computer! You check the
+system clock: 25 milliseconds until midnight. With that much time, you
+should be able to collect all fifty stars by December 25th.
+
+Collect stars by solving puzzles. Two puzzles will be made available
+on each day millisecond in the advent calendar; the second puzzle is
+unlocked when you complete the first. Each puzzle grants one star.
+Good luck!
+
+You're standing in a room with "digitization quarantine" written in
+LEDs along one wall. The only door is locked, but it includes a small
+interface. "Restricted Area - Strictly No Digitized Users Allowed."
+
+It goes on to explain that you may only leave by solving a captcha to
+prove you're not a human. Apparently, you only get one millisecond to
+solve the captcha: too fast for a normal human, but it feels like
+hours to you.
+
+The captcha requires you to review a sequence of digits (your puzzle
+input) and find the sum of all digits that match the next digit in the
+list. The list is circular, so the digit after the last digit is the
+first digit in the list.
+
+For example:
+
+    1122 produces a sum of 3 (1 + 2) because the first digit (1)
+    matches the second digit and the third digit (2) matches the
+    fourth digit.
+
+    1111 produces 4 because each digit (all 1) matches the next.
+
+    1234 produces 0 because no digit matches the next.
+
+    91212129 produces 9 because the only digit that matches the next
+    one is the last digit, 9.
+
+What is the solution to your captcha?
+
+Your puzzle answer was 1144.
+
+--- Part Two ---
+
+You notice a progress bar that jumps to 50% completion. Apparently,
+the door isn't yet satisfied, but it did emit a star as encouragement.
+The instructions change:
+
+Now, instead of considering the next digit, it wants you to consider
+the digit halfway around the circular list. That is, if your list
+contains 10 items, only include a digit in your sum if the digit 10/2
+= 5 steps forward matches it. Fortunately, your list has an even
+number of elements.
+
+For example:
+
+    1212 produces 6: the list contains 4 items, and all four digits
+    match the digit 2 items ahead.
+
+    1221 produces 0, because every comparison is between a 1 and a 2.
+
+    123425 produces 4, because both 2s match each other, but no other
+    digit has a match.
+
+    123123 produces 12.
+
+    12131415 produces 4.
+
+What is the solution to your new captcha?
+
+Your puzzle answer was 1194.
+
+Both parts of this puzzle are complete! They provide two gold stars: **