diff --git a/include/prime.h b/include/prime.h
index 8780dfd3..fd7b73ef 100644
--- a/include/prime.h
+++ b/include/prime.h
@@ -35,7 +35,7 @@ namespace alg {
 		if (n%2	== 0) return false;
 
 		unsigned sqrtn = sqrt(n);
-		for (unsigned int i = 2; i <= sqrtn; ++i) {
+		for (unsigned int i = 3; i <= sqrtn; i+=2) {
 			if (n % i == 0) {
 				return false;
 			}