Add information about the puzzle

day_01/info.md

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 https://adventofcode.com/2021/day/1
+
+## --- Day 1: Sonar Sweep ---
+
+You're minding your own business on a ship at sea when the overboard alarm goes
+off! You rush to see if you can help. Apparently, one of the Elves tripped and
+accidentally sent the sleigh keys flying into the ocean!
+
+Before you know it, you're inside a submarine the Elves keep ready for
+situations like this. It's covered in Christmas lights (because of course it
+is), and it even has an experimental antenna that should be able to track the
+keys if you can boost its signal strength high enough; there's a little meter
+that indicates the antenna's signal strength by displaying 0-50 stars.
+
+Your instincts tell you that in order to save Christmas, you'll need to get all
+fifty stars by December 25th.
+
+Collect stars by solving puzzles. Two puzzles will be made available on each
+day in the Advent calendar; the second puzzle is unlocked when you complete the
+first. Each puzzle grants one star. Good luck!
+
+As the submarine drops below the surface of the ocean, it automatically performs
+a sonar sweep of the nearby sea floor. On a small screen, the sonar sweep report
+(your puzzle input) appears: each line is a measurement of the sea floor depth
+as the sweep looks further and further away from the submarine.
+
+For example, suppose you had the following report:
+```
+199
+200
+208
+210
+200
+207
+240
+269
+260
+263
+```
+This report indicates that, scanning outward from the submarine, the sonar
+sweep found depths of `199`, `200`, `208`, `210`, and so on.
+
+The first order of business is to figure out how quickly the depth increases,
+just so you know what you're dealing with - you never know if the keys will get
+carried into deeper water by an ocean current or a fish or something.
+
+To do this, count the number of times a depth measurement increases from the
+previous measurement. (There is no measurement before the first measurement.)
+In the example above, the changes are as follows:
+```
+199 (N/A - no previous measurement)
+200 (increased)
+208 (increased)
+210 (increased)
+200 (decreased)
+207 (increased)
+240 (increased)
+269 (increased)
+260 (decreased)
+263 (increased)
+```
+In this example, there are 7 measurements that are larger than the previous
+measurement.
+
+How many measurements are larger than the previous measurement?
+
+
+## --- Part Two ---
+Considering every single measurement isn't as useful as you expected: there's
+just too much noise in the data.
+
+Instead, consider sums of a three-measurement sliding window. Again considering
+the above example:
+```
+199  A
+200  A B
+208  A B C
+210    B C D
+200  E   C D
+207  E F   D
+240  E F G
+269    F G H
+260      G H
+263        H
+```
+Start by comparing the first and second three-measurement windows. The
+measurements in the first window are marked A (`199`, `200`, `208`); their sum
+is `199 + 200 + 208 = 607`. The second window is marked B (`200`, `208`, `210`);
+its sum is `618`. The sum of measurements in the second window is larger than
+the sum of the first, so this first comparison increased.
+
+Your goal now is to count the number of times the sum of measurements in this
+sliding window increases from the previous sum. So, compare A with B, then
+compare B with C, then C with D, and so on. Stop when there aren't enough
+measurements left to create a new three-measurement sum.
+
+In the above example, the sum of each three-measurement window is as follows:
+
+```
+A: 607 (N/A - no previous sum)
+B: 618 (increased)
+C: 618 (no change)
+D: 617 (decreased)
+E: 647 (increased)
+F: 716 (increased)
+G: 769 (increased)
+H: 792 (increased)
+In this example, there are 5 sums that are larger than the previous sum.
+```
+
+Consider sums of a three-measurement sliding window. How many sums are larger
+than the previous sum?