#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

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

  if (m >= n) cout << 1 << endl;
  else {
    if (n % 2 == 0 && n / 2 == m) cout << 2 << endl;
    else cout << -1 << endl;
  }
}