#include<iostream>
#include<algorithm>
#include<cmath>
using namespace std;
int main() {
  int n,m;
  cin >> n >> m;
  if(n%2==0) {
      if(n<=m) {
          cout << 1 << endl;
      }
      else if(n>m&&n/2<=m) {
          cout << 2 << endl;
      }
      else {
          cout << -1 << endl;
      }
  }
  else {
      if(n<=m) {
          cout << 1 << endl;
      }
      else {
          cout << -1 << endl;
      }
  }
}