diff --git a/sum/foo.go b/sum/foo.go
deleted file mode 100644
index 3b01770..0000000
--- a/sum/foo.go
+++ /dev/null
@@ -1,3 +0,0 @@
-// +build !change
-
-package sum
diff --git a/sum/sum.go b/sum/sum.go
index 5e2b0d8..cfe2f18 100644
--- a/sum/sum.go
+++ b/sum/sum.go
@@ -2,6 +2,6 @@
 
 package sum
 
-func Sum(a, b int) int {
+func Sum(a, b int64) int64 {
 	return 0
 }
diff --git a/sum/sum_test.go b/sum/sum_test.go
index a31200c..0a65602 100644
--- a/sum/sum_test.go
+++ b/sum/sum_test.go
@@ -6,7 +6,7 @@ import (
 )
 
 type testCase struct {
-	a, b, sum int
+	a, b, sum int64
 }
 
 func TestSum(t *testing.T) {