#include <iostream>
#include <cstdio>

using namespace std;

int main(){
  int n,m;

  cin >> n >> m;

  if(n > m){
    if(n % 2 == 0 && n/2 <= m){
      printf("%d\n", 2);
    }else{
      puts("-1");
    }
  }else{
    int ans = 1;
    printf("%d\n",ans);
  }

  return 0;
}