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 switch (ch)
            {
                case 1:
                    c=Convert.ToInt32(a+b);
                    Console.WriteLine("sum of {0} and {1} = {2} ", a, b, c);
                    Console.ReadLine();
                    break;
                case 2:
                    c = Convert.ToInt32(a * b);
                    Console.WriteLine("Multiplication of {0} and {1} = {2} ", a, b, c);
                    Console.ReadLine();
                    break;
                case 3:
                    c = Convert.ToInt32(a / b);
                    Console.WriteLine("division of {0} and {1} = {2} ", a, b, c);
                    Console.ReadLine();
                    break;
                case 4:
                    c = Convert.ToInt32(a - b);
                    Console.WriteLine("differention of {0} and {1} = {2} ", a, b, c);
                    Console.ReadLine();
                    break;
            }