Console Calculator

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using ConsoleApplication6;
namespace ConsoleApplication6 {
    public class MojaKlasa {
        public
        const double const_PI = 3.14;
        public readonly double readOnly_PI;
        public MojaKlasa() {
            readOnly_PI = 3.14;
        }
    }
    public class MojKrug: MojaKlasa {
        public double PI = 3.14;
        public double op1;
        public double area() {
            return op1 * op1 * PI;
        }
    }
    class Kalkulator {
        public int op1;
        public int op2;
        public double add() {
            return op1 + op2;
        }
        public double sub() {
            return op1 - op2;
        }
        public double mul() {
            return op1 * op2;
        }
        public double div() {
            return op1 / op2;
        }
        public void Operands(int op1, int op2) {
            this.op1 = op1;
            this.op2 = op2;
        }
    }
    class Program {
        static void Main(string[] args) {
            MojKrug k = new MojKrug();
            Console.Write("Operand 1: ");
            k.op1 = Convert.ToInt32(Console.ReadLine());
            System.Console.WriteLine(k.area());
            Kalkulator c = new Kalkulator();
            Console.Write("Operand 1: ");
            c.op1 = Convert.ToInt32(Console.ReadLine());
            Console.Write("Operand 2: ");
            c.op2 = Convert.ToInt32(Console.ReadLine());
            Console.Write("Operator : ");
            string oper = Console.ReadLine();
            switch (oper) {
                case "+":
                    Console.WriteLine(c.add());
                    break;
                case "-":
                    Console.WriteLine(c.sub());
                    break;
                case "*":
                    Console.WriteLine(c.mul());
                    break;
                case "/":
                    Console.WriteLine(c.div());
                    break;
            }
        }
    }
}