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

}