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