public void Add()
 {
     Assert.AreEqual(0, Calculator.Add(0, 0));
     Assert.AreEqual(4, Calculator.Add(2, 2));
     Assert.AreEqual(8, Calculator.Add(2, 6));
 }
 public void Subtract()
 {
     Assert.AreEqual(6, Calculator.Subtract(10, 4));
 }