#include<iostream>
using namespace std;

int main(){
    int n,m,result;
    cin >> n;
    cin >> m;

    if((n%2) == 1){
        if(n <= m){
            result = 1;
        }else{
            result = -1;
        }
    }else{
        if(n <= m){
            result = 1;
        }else if((n/2) <= m){
            result = 2;
        }else{
            result = -1;
        }
    }

    cout << result << endl; 

    return 0;
}