// calculate the number of non-empty segment which sums to a value >= 0
int sol(const vector<int>& a) {
    int ans = 0;
    int acc = 0;
    // for definition of add() and sum(), please refer to
    // http://www.csie.ntnu.edu.tw/~u91029/Sequence2.html#4
    add(0, 1);
    for (auto v: a) {
        acc += v;
        ans += sum(acc);
        add(acc, 1);
    }
    return ans;
}