Abstract overriding in console apps

abstract class Abstract {

public abstract void mul(int x);
public void add(int x, int y)
{
Console.WriteLine("add is: "+(x + y));
}
}
class overrideabstract : Abstract
{
public override void mul(int a)
{
Console.WriteLine("mul is: "+a*a);
}
public static void Main()
{
overrideabstract ovra = new overrideabstract();
ovra.add(4, 5);
ovra.mul(3);
Console.ReadLine();
}
}

Note: An abstract class can implement from another abstract class. and a class can implement only one abstract class at a time.