what is the correct way to implement open-closed principle in C#?

Question

namespace Test
 {
 class TestOCP
 {
   ADD();
   SUB();
 }
 }

Here are two methods, ADD() and SUB(), and this gone through unit test, and according to OCP, how should > I add MUL() method in the class,

Please help me, I have tried this,

interface ICalculate
    {
        int CalculateTotal(int a, int b);
    }
    class CalculateAddition : ICalculate
    {
        public int CalculateTotal(int a, int b)
        {
            return a + b;
        }
    }
    class CalculateSubstraction : ICalculate
    {
        public int CalculateTotal(int a, int b)
        {
            return a - b;
        }
    }
    class CalculateMultiplication : ICalculate
    {
        public int CalculateTotal(int a, int b)
        {
            return a * b;
        }
    }