using static System.Console;

using System;

​

public class Program {

    public static void Main() {

        ContaPoupanca poupanca = new ContaPoupanca();

        poupanca.CalculaInvestimento();

        Conta conta = new ContaPoupanca();

        conta.CalculaInvestimento();

    }

}

​

public class Conta {

    public virtual void CalculaInvestimento() { throw new NotImplementedException(); }

}

public class ContaPoupanca : Conta {

    public override void CalculaInvestimento() { WriteLine("ok"); }

}

​