#include <iostream>

using namespace std;

int main() {
  int n, m;

  cin >> n, m;
  if ( n % 2 == 0 ) {

    if ( m > (n/2) ) {
      cout << 2 << endl;
    } else {
      cout << -1 << endl;
    }

  } else {

    if ( n < m ) {
      cout << 1 << endl;
    } else {
      cout << -1 << endl;
    }

  }

  return 0;

}