C# Abstract Class

public abstract class CalcAb
{
  public abstract int Add(int i, int j);

  public int Subtract(int i, int j)
  {
    return i - j;
  }
}

public class Calc : CalcAb
{
  public override int Add(int i, int j)
  {
    return i + j;
  }
}

protected void Page_Load(object sender, EventArgs e)
{
  Calc c = new Calc();
  Response.Write(c.Add(78, 56));
  Response.Write("<br/>");
  Response.Write(c.Subtract(78, 67));
}