using System;

public class Program

{

    public static void Main()

    {

        var cpBook = new GradeBook();

        cpBook.AddToGrade(70);

        Console.WriteLine(cpBook.Grade);

    }

}

​

public class GradeBook

{

    private int _grade;

    public int Grade

    {

        get { return _grade; }

        set { _grade = value; }

    }

    public void AddToGrade(int input)

    {

        _grade += input;

    }

}