Exception While Debugging WCF service in Visual Studio

[ServiceContract]
    public interface IService2
    {
        [OperationContract]
        Int32 ADD(Int32 num1 , Int32 num2);

        [OperationContract]
        Int32 MULTIPLY(Int32 num1, Int32 num2);

        [OperationContract]
        Int32 SUBTRACT(Int32 num1, Int32 num2);

        [OperationContract]
        Int32 DIVISION(Int32 num1, Int32 num2);

public Int32 ADD(Int32 num1, Int32 num2)
       {
           return num1 + num2;
       }

       public Int32 MULTIPLY(Int32 num1, Int32 num2)
       {
           return num1 - num2;
       }


       public Int32 SUBTRACT(Int32 num1, Int32 num2)
       {
           return num1 * num2;
       }

       public Int32 DIVISION(Int32 num1, Int32 num2)
       {
           if (num2.Equals(0))
           {
               return num1;
           }
           else
           {
               return num1 / num2;
           }
       }