diff --git a/calculator.js b/calculator.js
index caf34db..948bd35 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);
 }
 
@@ -11,5 +11,21 @@ function substract(num1, num2){
   return num1 - num2;
 }
 
+function multiply(num1, num2){
+	return num1 * num2;
+}
+
+function divide(num1, num2){
+	return num1 / num2;
+}
+
+function mod(num1, num2){
+	return num1 % num2;
+}
+
 module.exports.add = add;
+module.exports.substract = substract;
+module.exports.multiply = multiply;
+module.exports.divide = divide;
+module.exports.mod = mod;
 module.exports.isNumber = isNumber;
diff --git a/index.html b/index.html
index cc6a9d9..d5910f8 100644
--- a/index.html
+++ b/index.html
@@ -22,6 +22,9 @@ <h1>Silly Calculator</h1>
     <select id="operator" size="1">
       <option value="add">+</option>
       <option value="substract">-</option>
+      <option value="multiply">*</option>
+      <option value="divide">/</option>
+      <option value="mod">%</option>
     </select>
     <input type="text" id="num2" value="1" />
     <div>=</div>
diff --git a/main.js b/main.js
index 957a234..2f665ac 100644
--- a/main.js
+++ b/main.js
@@ -5,19 +5,36 @@ window.onload = function(){
     if (!isNumber(num1Str) || !isNumber(num2Str)){
       alert("Some of the input is not a number!")
       document.getElementById('ans').value = "ERROR"
-      return
+      return;
     }
-    var num1 = parseFloat(num1Str)
-    var num2 = parseFloat(num2Str)
+    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;
+
+      case "mod":
+        document.getElementById('ans').value = mod(num1, num2);
+      break;
+
+      default:
+        alert("Bad operator!");
+      break;
     }
   }
 }
diff --git a/test/first_test.js b/test/first_test.js
new file mode 100644
index 0000000..38ef741
--- /dev/null
+++ b/test/first_test.js
@@ -0,0 +1,9 @@
+// first test
+var assert = require("assert");
+var foo = "bar";
+
+describe("First", function() {
+	it("Test", function() {
+		assert.equal(foo, "bar");
+	});
+});
\ No newline at end of file
diff --git a/test/test_calculator.js b/test/test_calculator.js
index 13f6205..19ebc50 100644
--- a/test/test_calculator.js
+++ b/test/test_calculator.js
@@ -2,9 +2,64 @@ var assert = require('assert');
 var cal = require('../calculator.js')
 
 describe('Calculator', function(){
+
+  describe('isNumber', function(){
+  	it('input is number', function(){
+  	  assert.equal(true, cal.isNumber(513));
+ 	}),
+
+ 	it('input is not number', function(){
+ 	  assert.equal(false, cal.isNumber("asd12"));
+ 	})
+  }),
+
   describe('add', function(){
-    it('add', function(){
+    it('add_positive', function(){
       assert.equal('2', cal.add(1, 1));
+    }),
+
+    it('add_negative', function(){
+      assert.equal('-4', cal.add(1, -5));
+    })
+  }),
+
+  describe('substract', function(){
+    it('substract_positive', function(){
+      assert.equal('2', cal.substract(3, 1));
+    }),
+
+    it('substract_negative', function(){
+      assert.equal('6', cal.substract(-1, -7));
+    })
+  }),
+
+  describe('multiply', function(){
+    it('multiply_positive', function(){
+      assert.equal('10', cal.multiply(2, 5));
+    }),
+
+    it('multiply_negative', function(){
+      assert.equal('-15', cal.multiply(-3, 5));
+    })
+  }),
+
+  describe('divide', function(){
+    it('divide_positive', function(){
+      assert.equal('2', cal.divide(4, 2));
+    }),
+
+    it('divide_negative', function(){
+      assert.equal('-5', cal.divide(20, -4));
+    })
+  }),
+
+  describe('mod', function(){
+    it('mod_positive', function(){
+      assert.equal('2', cal.mod(5, 3));
+    }),
+
+    it('mod_negative', function(){
+      assert.equal('-2', cal.mod(-5, -3));
     })
   })
-})
+});