#include <bits/stdc++.h>
#define int long long
using namespace std;
const int MOD = 1000000007;
using Graph = vector<vector<int>>;

signed main() {
  int N, M;
  cin >> N >> M;
  
  if( M >= N ){
    cout << 1 << endl;
    return 0;
  }else if( N%2 == 0 && M >= N/2 ) cout << 2 << endl;
  else cout << -1 << endl;
}