private int compute( String s1, String s2, String op ) {
		int		a  =  Integer.parseInt(s1);
		int		b  =  Integer.parseInt(s2);
		int		ret = 0;
		switch (op) {
		case "+":
			ret = a + b;
			break;
		case "-":
			ret = a - b;
			break;
		case "*":
			ret = a * b;
			break;
		case "/":
			ret = a / b;
			break;
		default:
			break;
		}
		return ret;
	}