diff --git a/package.json b/package.json
index b513f3a..c1f9bb4 100644
--- a/package.json
+++ b/package.json
@@ -4,7 +4,7 @@
   "description": "Learning GitHub Actions",
   "main": "dist/index.js",
   "scripts": {
-    "build": "tsc src/index.ts --module commonjs --outDir dist",
+    "build": "tsc src/index.ts --module commonjs --outDir dist --lib es2015",
     "test": "jest"
   },
   "repository": {
diff --git a/src/index.ts b/src/index.ts
index c68b57a..a3c7f3d 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,3 +1,7 @@
 export const add = (a: number, b: number) => {
   return a + b;
 };
+
+export const subtract = (a: number, b: number) => {
+  return a - b;
+};