import java.util.*;

public class Bin2Dec {
	static Scanner input = new Scanner(System.in);

	public static void main(String[] args) {

		try {
			System.out.println("Enter binary number");
			String s = input.next();
			System.out.println("Decimal number = " + bin2Dec(s));
		} catch (NumberFormatException e) {
			System.out.println(e);
		}

	}

	public static int bin2Dec(String num)   {
		 int sum = 0;
		    for(int i = num.length()-1, p = 0; i >= 0; i--, p++) {
		        char c = num.charAt(i);
		        if(c!= '1' && c != '0')
		            throw new NumberFormatException ("Error you must insetrt binary number only.");
	            if(c == '1')
	               sum += Math.pow(2, p);
		    }
	            return sum;
		}
	

	}