How Can I Use Abstract In The Interface?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace InplicitWithAbstract
{
    class Program
    {
        static void Main(string[] args)
        {
            B b = new B();
            int r2=b.m2(8,2);
            Console.WriteLine(r2);
            A a = b; //UPCASTING
            int r1=a.m1(4,7);
            Console.WriteLine(r1);

            Console.ReadLine();
        }
    }

        public abstract interface I1
        {
            int m1(int x, int y);
            abstract int m2(int a, int b);
        }


    public abstract class A : I1
    {
        public int m1(int x, int y)
        {
            return x + y;
        }
    }
    public class B : A
    {
        public override int m2(int a, int b)
        {
            return a - b;
        }
    }
}