int preCalc(string line) {
    Stack stack(line.length());
    for (int i = line.length() - 1; i >= 0; i--) {
        if (line[i] >= '0' and line[i] <= '9') {
            stack.push(line[i] - '0');
        }
        else {
            int x, y;
            switch (line[i])
            {
            case '+':
                x = stack.peek();
                stack.pop();
                y = stack.peek();
                stack.pop();
                stack.push(x + y);
                break;
            case '-':
                x = stack.peek();
                stack.pop();
                y = stack.peek();
                stack.pop();
                stack.push(x - y);
                break;
            case '*':
                x = stack.peek();
                stack.pop();
                y = stack.peek();
                stack.pop();
                stack.push(x * y);
                break;
            case '/':
                x = stack.peek();
                stack.pop();
                y = stack.peek();
                stack.pop();
                stack.push(x / y);
                break;
            default:
                break;
            }
        }
    }
    return stack.peek();
}