String changeSign() {
	    minusOne = minusOne * (-1);
	    if (enterX) {
	        x = 0 - result;
	        cleanX = false;
	        prevOpn = "";
	        return format(x);
	    } else if (equalCalled) {
	        y = 0 - result;
	        cleanY = false;
	        return format(y);
	    } else {
            y = 0 - result;
            cleanY = false;
            return format(y);
	    }
	}