public static int evaluate(String in) {
        StkInt stack = new StkInt();
        String opers = "+-*/";
        for (int a, b, i = 0; i < in.length(); i++) {
            if (opers.indexOf(in.charAt(i)) == -1) {
                stack.push(in.charAt(i) - 48);
            } else {
                b = stack.pop();
                a = stack.pop();
                if (in.charAt(i) == '+') {
                    a = a + b;
                } else if (in.charAt(i) == '-') {
                    a = a - b;
                } else if (in.charAt(i) == '*') {
                    a = a * b;
                } else if (in.charAt(i) == '/') {
                    a = a / b;
                }
                stack.push(a);
            }
        }
        return stack.pop();
    }