long long fibonacci(int index) {
    if (index < 0) throw runtime_error("invalid parameter with negative index");

    if (index == 0) return 0;
    if (index == 1) return 1;

    long long pre = 0, tail = 1, res = 1;
    for (int i = 1; i < index; i++) {
        res  = pre + tail;
        pre  = tail;
        tail = res;
    }

    return res;
}