diff --git a/day_25/first.rb b/day_25/first.rb
new file mode 100644
index 0000000..aa162ef
--- /dev/null
+++ b/day_25/first.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+d_pub = 17_607_508
+c_pub = 15_065_270
+modulo = 20_201_227
+
+subject_mum = 7
+val = 1
+
+0.step do |i|
+  (puts(c_pub.pow(i, modulo)); break) if val == d_pub
+  val *= subject_mum
+  val %= modulo
+end