diff --git a/Algo/B2/Exercises/Repetitive tutorial.md b/Algo/B2/Exercises/Repetitive tutorial.md
index c320b2e..cddd208 100644
--- a/Algo/B2/Exercises/Repetitive tutorial.md	
+++ b/Algo/B2/Exercises/Repetitive tutorial.md	
@@ -214,6 +214,11 @@ def prime(n: int) -> int:
 		raise Exception("prime: n<= 1")
 	elif n % 2 == 0:
 		return n == 2
+	else:
+		d = 3
+		while d*d <= n and n % d!= 0:
+			d += 2
+		return d*d > n
 ## Exercise 3.3
 ```python
 def quotient(a: int, b: int) -> int: