#include <bits/stdc++.h>   using namespace std;   const int N = 1e5 + 7;   long long eq(long long start, long long k) {     long long num = k - (start - 1);     return ((num * (start + k)) / 2) - (num - 1);   }   void solve() {     long long n, k;     cin >> n >> k;     long long q = eq(2, k);     if (n == 1)       cout << 0 <<   n ;     else if (n <= k)       cout << 1 <<   n ;     else if (q < n)       cout << -1 <<   n ;     else {       long long en = k;       long long st = 2;       long long res = -1;       while (st < en && st > 1) {         long long mid = st + en >> 1;         if (eq(mid, k) < n) {           en = mid;         } else {           st = mid + 1;           res = mid;         }       }       res--;       cout << k - res <<   n ;     }   }   int main() {     ios::sync_with_stdio(0), cin.tie(nullptr), cout.tie(nullptr);     int t = 1;     while (t--) {       solve();     }     return 0;   }   