use of constructor in abstract class

Question

when i wrote below program it gave me exception as

Error1'abstractclassconntruct.program' does not implement inherited abstract member 'abstractclassconntruct.A.area()'F:\net\abstractclassconntruct\abstractclassconntruct\Program.cs4315abstractclassconntruct

can anyone plz help me to sort out it???

using System;

namespace abstractclassconntruct

{

    public abstract class A

    {

        protected double x = 10;

        protected double y = 7;

        public A()

        {

            

        }

        public A(double x, double y)

        {

            

            this.x = x;

            this.y = y;

        }

        public abstract double area();

    }

    public class square : A

    {

        public square()

        { }

        public override double area()

        {

            return x * x;

        }

    }

        public class rectangle:A

    {

        public rectangle(double x,double y): base (x,y)

        { }

        public override double area()

        {

            return x * y;

        }

    

    }

        class program : A

        {

            static void Main(string[] args)

            {

                square s = new square();

                double areacal = s.area();

                rectangle r = new rectangle(10, 5);

                double areaca = r.area();

                Console.ReadKey();

            }

        }

    

    }