int twoStacks(int x, stack<int> a, stack<int> b) {

    if(x < 0) {
        return -1;
    }
    int a_top = -1, b_top = -1, m1 = 0, m2 = 0;
    if(!a.empty()) {    // Pop A
        a_top = a.top();
        a.pop();
        m1 = 1 + twoStacks(x - a_top, a, b);
    }
    if(!b.empty()) {    // Pop B
        b_top = b.top();
        b.pop();
        if(a_top != -1) {
            a.push(a_top);
        }
        m2 = 1 + twoStacks(x - b_top, a, b);
    }
    return max(m1, m2);
}