boolean checkValidFeedback(String feedback){
    try {
	    check(feedback.length() == 4);
	    check(feedback.charAt(1) == 'b');
	    check(feedback.charAt(3) == 'w');
	    int b = feedback.charAt(0) - '0';
	    int w = feedback.charAt(2) -'0';
	    check(b <= CL && b >= 0);
	    check(w <= CL && w >= 0);
	    check(b+w <=CL && b+w >= 0);
    } catch (Exception e) {
	    return false;
    }
    return true;
    
  }