class Test {

  public static void main(String args[]) {
    Scanner scanner = new Scanner(System.in);
    System.out.println("Enter binary number");
    String input = scanner.next();
    // validate : 0 or 1 only
    for (int pos = 0; pos < input.length(); pos++) {
      char c = input.charAt(pos);
      if (c != '0' && c != '1') {
        System.out.println("Not binary");
        System.exit(1);
      }
    }
    System.out.println("Input valid");
    int result = 0;
    int factor = 1; // place value
    int length = input.length();
    // go through input, starting at the right
    for (int pos = length - 1; pos > -1; pos--) {
      char c = input.charAt(pos);
      if (c == '1') {
        result += factor;
      }
      factor *= 2; //  next place value
    }
    System.out.println("In decimal " + result);
  }
}