diff --git a/calculator.js b/calculator.js
index caf34db..a2280d6 100644
--- a/calculator.js
+++ b/calculator.js
@@ -1,15 +1,31 @@
-function isNumber(numStr){
-  var re = /^[0-9\.]+$/; 
+function isNumber(numStr)
+{
+  var re = /^-?[0-9]+(\.[0-9]+)?$/; 
   return re.test(numStr);
 }
 
-function add(num1, num2){
+function add(num1, num2)
+{
   return num1 + num2;
 }
 
-function substract(num1, num2){
+function substract(num1, num2)
+{
   return num1 - num2;
 }
 
+function multiple(num1, num2)
+{
+  return num1 * num2;
+}
+
+function division(num1, num2)
+{
+  return num1 / num2;
+}
+
 module.exports.add = add;
+module.exports.substract = substract;
+module.exports.multiple = multiple;
+module.exports.division = division;
 module.exports.isNumber = isNumber;
diff --git a/index.html b/index.html
index cc6a9d9..e755246 100644
--- a/index.html
+++ b/index.html
@@ -22,6 +22,8 @@ <h1>Silly Calculator</h1>
     <select id="operator" size="1">
       <option value="add">+</option>
       <option value="substract">-</option>
+      <option value="multiple">×</option>
+      <option value="division">÷</option>
     </select>
     <input type="text" id="num2" value="1" />
     <div>=</div>
diff --git a/main.js b/main.js
index 957a234..33d4301 100644
--- a/main.js
+++ b/main.js
@@ -12,11 +12,13 @@ window.onload = function(){
     var operator = document.getElementById('operator').value;
     if (operator == "add"){
       document.getElementById('ans').value = add(num1, num2);
-    }
-    else if (operator == "substract"){
+    } else if (operator == "substract") {
       document.getElementById('ans').value = substract(num1, num2);
-    }
-    else {
+    } else if (operator == "multiple") {
+      document.getElementById('ans').value = multiple(num1, num2);
+    } else if (operator == "division") {
+      document.getElementById('ans').value = division(num1, num2);
+    } else {
       alert("Bad operator!")
     }
   }
diff --git a/test/test_calculator.js b/test/test_calculator.js
index 13f6205..c4cf58e 100644
--- a/test/test_calculator.js
+++ b/test/test_calculator.js
@@ -3,8 +3,109 @@ var cal = require('../calculator.js')
 
 describe('Calculator', function(){
   describe('add', function(){
-    it('add', function(){
-      assert.equal('2', cal.add(1, 1));
+    it('0~9 + 0~9', function(){
+      for (var i = 0; i < 10; i++) {
+        for (var j = 0; j < 10; j++) {
+          assert.equal(i+j, cal.add(i, j));
+        }
+      }
+    })
+  })
+
+  describe('add', function(){
+    it('1000 random case', function(){
+      for (var i = 0; i < 1000; i++) {
+          var x = Math.random();
+          var y = Math.random();
+          assert.equal(x+y, cal.add(x, y));
+      }
+    })
+  })
+
+  describe('substract', function(){
+    it('0~9 - 0~9', function(){
+      for (var i = 0; i < 10; i++) {
+        for (var j = 0; j < 10; j++) {
+          assert.equal(i-j, cal.substract(i, j));
+        }
+      }
+    })
+  })
+
+  describe('substract', function(){
+    it('1000 random case', function(){
+      for (var i = 0; i < 1000; i++) {
+          var x = Math.random();
+          var y = Math.random();
+          assert.equal(x-y, cal.substract(x, y));
+      }
+    })
+  })
+
+  describe('multiple', function(){
+    it('0~9 * 0~9', function(){
+      for (var i = 0; i < 10; i++) {
+        for (var j = 0; j < 10; j++) {
+          assert.equal(i*j, cal.multiple(i, j));
+        }
+      }
+    })
+  })
+
+  describe('multiple', function(){
+    it('1000 random case', function(){
+      for (var i = 0; i < 1000; i++) {
+          var x = Math.random();
+          var y = Math.random();
+          assert.equal(x*y, cal.multiple(x, y));
+      }
+    })
+  })
+
+  describe('division', function(){
+    it('1~9 / 1~9', function(){
+      for (var i = 1; i < 10; i++) {
+        for (var j = 1; j < 10; j++) {
+          assert.equal(i/j, cal.division(i, j));
+        }
+      }
+    })
+  })
+
+  describe('division', function(){
+    it('1000 random case', function(){
+      for (var i = 0; i < 1000; i++) {
+          var x = Math.random();
+          var y = Math.random();
+          assert.equal(x/y, cal.division(x, y));
+      }
+    })
+  })
+
+  describe('Is number?', function(){
+    it('Positive', function(){
+        assert.equal(true, cal.isNumber('123.456'));
+        assert.equal(true, cal.isNumber('-789.012'));
+        assert.equal(true, cal.isNumber('345'));
+        assert.equal(true, cal.isNumber('-678'));
+        assert.equal(true, cal.isNumber('0.901'));
+        assert.equal(true, cal.isNumber('-0.234'));
+        assert.equal(true, cal.isNumber('0567'));
+        assert.equal(true, cal.isNumber('-0890'));
+    })
+  })
+
+  describe('Is number?', function(){
+    it('Negative', function(){
+        assert.equal(false, cal.isNumber('.12'));
+        assert.equal(false, cal.isNumber('+123.456'));
+        assert.equal(false, cal.isNumber('0.'));
+        assert.equal(false, cal.isNumber('123.456.789'));
+        assert.equal(false, cal.isNumber('1e10'));
+        assert.equal(false, cal.isNumber('1e-10'));
+        assert.equal(false, cal.isNumber('-1e10'));
+        assert.equal(false, cal.isNumber('-1e-10'));
+        assert.equal(false, cal.isNumber('0xabcdef'));
     })
   })
 })