#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  cin >> n >> m;
  int res = 0;
  bool no = 0;
  int i = 0;
  while (1) {
    i += m;
    if (i > n) {
      no = 1;
      break;
    }
    if ((2 * i) >= n) {
      res = i;
      break;
    }
  }
  if (no)
    cout << "-1" << endl;
  else
    cout << res;
}
