#include <bits/stdc++.h>   using namespace std;   int dx[] = {0, 1, 0, -1, -1, -1, 1, 1};   int dy[] = {1, 0, -1, 0, 1, -1, 1, -1};   int main() {     ios::sync_with_stdio(false);     long long n, k;     cin >> n >> k;     if (n == 1)       cout << 0;     else if (n <= k)       cout << 1;     else {       n--;       long long mx = k * (k - 1) / 2;       if (n > mx) {         cout << -1;         return 0;       }       long long l = 1, r = k - 1, mid, res;       while (l <= r) {         mid = (l + r) >> 1;         long long sum = mx - mid * (mid - 1) / 2;         if (sum > n)           l = mid + 1;         else           res = mid, r = mid - 1;       }       long long sum = mx - res * (res - 1) / 2;       res = k - res;       if (sum < n) res++;       cout << res << endl;     }     return 0;   }   