Codeforces Round #176 (Div. 2)

B - Pipeline

#include <iostream>using namespace std;long long n, k;int main() {    cin >> n >> k;    long long l = 0, r = k, mid;    while (l < r)     {        mid = (l + r) / 2;        long long s = mid * k - (mid - 1) * (mid + 2) / 2;        if (s < n)             l = mid + 1;        else            r = mid;    }    mid = (l + r) / 2;    if (mid * k - (mid - 1) * (mid + 2) / 2 < n)        cout << -1 << endl;    else        cout << mid << endl;    return 0;}