diff --git a/chapter_02/birthday.py b/chapter_02/birthday.py
index 796c848..dc2f4f1 100644
--- a/chapter_02/birthday.py
+++ b/chapter_02/birthday.py
@@ -31,7 +31,7 @@
         match = 0
         b = np.random.randint(0,364,m)
         for i in range(m):
-            for j in range(m):
+            for j in range(i, m): # starting from i to avoir comparing the same pair a second time
                 if (i != j) and (b[i] == b[j]):
                     match += 1
         if (match != 0):