#include <iostream>
#include <stdio.h>
#include <string>
#include <vector>
#include <cmath>
#include <algorithm>
#define MP make_pair
using ll = long long;
using namespace std;

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

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