diff --git a/calculator.js b/calculator.js
index caf34db..e19bf10 100644
--- a/calculator.js
+++ b/calculator.js
@@ -1,5 +1,5 @@
 function isNumber(numStr){
-  var re = /^[0-9\.]+$/; 
+  var re = /^-?([0-9\.])+$/; 
   return re.test(numStr);
 }
 
@@ -11,5 +11,16 @@ function substract(num1, num2){
   return num1 - num2;
 }
 
+function multiply(num1, num2){
+	return num1 * num2;
+}
+
+function divide(num1, num2){
+	return num1 / num2;
+}
+
 module.exports.add = add;
+module.exports.substract = substract;
+module.exports.multiply = multiply;
+module.exports.divide = divide;
 module.exports.isNumber = isNumber;
diff --git a/index.html b/index.html
index cc6a9d9..ec5f210 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="multiply">x</option>
+			<option value="divide">/</option>
     </select>
     <input type="text" id="num2" value="1" />
     <div>=</div>
diff --git a/main.js b/main.js
index 957a234..1f1e3b7 100644
--- a/main.js
+++ b/main.js
@@ -10,14 +10,21 @@ window.onload = function(){
     var num1 = parseFloat(num1Str)
     var num2 = parseFloat(num2Str)
     var operator = document.getElementById('operator').value;
-    if (operator == "add"){
-      document.getElementById('ans').value = add(num1, num2);
-    }
-    else if (operator == "substract"){
-      document.getElementById('ans').value = substract(num1, num2);
-    }
-    else {
-      alert("Bad operator!")
-    }
+		switch (operator) {
+			case "add":
+				document.getElementById('ans').value = add(num1, num2);
+				break;
+			case "substract":
+				document.getElementById('ans').value = substract(num1, num2);
+				break;
+			case "multiply":
+				document.getElementById('ans').value = multiply(num1, num2);
+				break;
+			case "divide":
+				document.getElementById('ans').value = divide(num1, num2);
+				break;
+			default:
+				alert("Bad operator!");
+		}
   }
 }
diff --git a/test/test_calculator.js b/test/test_calculator.js
index 13f6205..0da2f81 100644
--- a/test/test_calculator.js
+++ b/test/test_calculator.js
@@ -7,4 +7,25 @@ describe('Calculator', function(){
       assert.equal('2', cal.add(1, 1));
     })
   })
+  describe('substract', function(){
+    it('substract', function(){
+      assert.equal('3', cal.substract(4, 1));
+    })
+  })
+  describe('multiply', function(){
+    it('', function(){
+      assert.equal('2', cal.multiply(1, 2));
+    })
+  })
+  describe('divide', function(){
+    it('divide', function(){
+      assert.equal('2', cal.divide(2, 1));
+    })
+  })
+	describe('isNumber', function(){
+		it('isNumber', function(){
+			assert.equal('1', cal.isNumber(1));
+			assert.equal('0', cal.isNumber('YOLO'));
+		})
+	})
 })