#include<bits/stdc++.h>

using namespace std;

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