abstract class A<T>
{
    public abstract void Foo(T x);
}

abstract class B<T, S> : A<T>
{
    public virtual void Foo(S x){}
}

abstract class C<T, S> : B<T, S>
{
    public override void Foo(T x) {}
}

class D : C<int, int> // here
{
    public override void Foo(int x) {}
}