diff --git a/calculator.js b/calculator.js
index caf34db..081b0ef 100644
--- a/calculator.js
+++ b/calculator.js
@@ -1,5 +1,5 @@
 function isNumber(numStr){
-  var re = /^[0-9\.]+$/; 
+    var re = /^[+-]?[0-9]+\.{0,1}[0-9]*$/;
   return re.test(numStr);
 }
 
diff --git a/test/test_calculator.js b/test/test_calculator.js
index 13f6205..5877a9d 100644
--- a/test/test_calculator.js
+++ b/test/test_calculator.js
@@ -1,10 +1,17 @@
 var assert = require('assert');
 var cal = require('../calculator.js')
 
-describe('Calculator', function(){
-  describe('add', function(){
-    it('add', function(){
-      assert.equal('2', cal.add(1, 1));
+describe('Calculator', function () {
+    describe('add_positive', function () {
+        it('add', function () {
+            assert.equal('2', cal.add(1, 1));
+
+        })
+    })
+    describe('add_nagative', function () {
+        it('add', function () {
+            assert.equal('-2', cal.add(-1, -1));
+
+        })
     })
-  })
 })