#include <bits/stdc++.h>
using namespace std;
int main() {
  int z, n, m;
  while (cin >> n >> m) {
    if (m > n)
      z = -1;
    else {
      if (n % 2 == 0)
        z = n / 2;
      else
        z = (n / 2) + 1;
      while (z % m != 0) {
        z++;
      }
    }
    cout << z;
  }
  return 0;
}
