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diff --git a/2020/aoc2020-d25.py b/2020/aoc2020-d25.py
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+#advent of code 2020
+#day 25
+#I made a mistake by thinking that subject number is an unknow
+#while in reality it actually is =7 as in puzzle description
+
+def FindNumbers(pubkey):
+	S = 7;
+	L = 0;
+	v = 1;
+	while True:
+		L += 1;
+		v *= S;
+		v = v%20201227;
+		if v == pubkey:
+			return L; #gives loop number
+
+PuzzleInput = open("25.in","r").read().split("\n");
+cardkey = int(PuzzleInput[0]);
+doorkey = int(PuzzleInput[1]);
+#encryption key is the same 
+#whether we use loop of card + subject of door
+#or loop of door + subject of card
+encryption = 1;
+for l in range(FindNumbers(cardkey)):
+	encryption *= doorkey;
+	encryption = encryption%20201227;
+
+part1 = encryption; 
+print("part 1 =", part1);