#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
typedef long long ll;
#define int long long

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