diff --git a/day25.cr b/day25.cr
new file mode 100644
index 0000000..88f82ef
--- /dev/null
+++ b/day25.cr
@@ -0,0 +1,28 @@
+require "advent"
+INPUT = input(2020, 25).lines.map(&.to_i64) 
+
+def transform(s)
+  i = 1_i64
+  c = 0
+  loop do
+    c += 1
+    i = (i * s) % 20201227
+    yield i, c
+  end
+end
+
+def find_size(s, goal)
+  transform(s) do |n, c|
+    return c if n == goal
+  end
+end
+
+def part1(input)
+  goal = find_size(7, input[0])
+  puts goal
+  transform(input[1]) do |n, c|
+    return n if c == goal
+  end
+end
+
+puts part1(INPUT.clone)