func_code_string: #include <bits/stdc++.h>   using namespace std;   long long n, k;   bool check(long long mid) {     long long sum = (mid + k - 1) * (k - mid) / 2;     return (sum >= n - 1);   }   int main() {     scanf( %I64d%I64d , &n, &k);     if (n == 1) {       printf( 0 );       return 0;     }     long long l = 1, r = k - 1, mid;     while (l < r) {       mid = (l + r + 1) >> 1;       if (check(mid))         l = mid;       else         r = mid - 1;     }     if ((l + k - 1) * (k - l) / 2 >= n - 1)       printf( %I64d , k - l);     else       printf( -1 );   }   