using System;

public class Program

{

    public static bool IsBinary(int N)

    {

        while (N!=0)

        {

            if (N%10>1)

                return false;

            N=N/10;

        }

        return true;

    }

    public static double DecNumber(int n)

    {

        int i=0,D;

        double sum=0;

        while (n!=0)

        {

            D=n%10;

            n=n/10;

            sum=sum+D*Math.Pow(2,i);

            i++;

        }

        return sum;

    }

public static void Main()

    {int n;

         Console.WriteLine("enter a binary number: ");

         n=int.Parse(Console.ReadLine());

        if (IsBinary(n))

        Console.WriteLine(DecNumber(n));

        else 

        Console.WriteLine("error");

    }

}

​