#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<string>
#include<sstream>
#include<cmath>
#include<numeric>
#include<map>
#include<stack>
#include<queue>
#include<list>
using namespace std;
int inf = 1000000000;

int main(void) {

  int n, m;
  cin >> n >> m;
  if( n <= m ){
    cout << 1 << endl;
    return 0;
  }
  if( n % 2 == 1 || m == 1 || n > 2 * m ){
    cout << -1 << endl;
    return 0;
  }

  cout << 2 << endl;



  return 0;
}

// EOF