#include <bits/stdc++.h>

using namespace std;

int main(void) {
  int n, m;
  cin >> n >> m;

  if (m >= n) {
    cout << 1 << endl;
  } else if (n & 1) {
    cout << -1 << endl;
  } else {
    cout << (m >= n / 2 ? 2 : -1) << endl;
  }

  return 0;
}