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;
}