#include <bits/stdc++.h>
using namespace std;
int main(){
    int n,m;
    cin>>n>>m;
    if(n>m){
        cout<<-1<<endl;
    }
    else if(n==m) cout<<1<<endl;
    else{
        if(n>=(m+1)/2){
            cout<<2<<endl;
        }
        else cout<<"-1"<<endl;
    }
    return 0;
}