#include <iostream>
#include <vector>
#include <queue>
#include <string>

using namespace std;

int main(void){
    long long n, m;
    cin >> n >> m;


    if(n <= m){
        cout << "1" << endl;
    }else{
        if(n%2 == 0 &&  n/2 <= m){
            cout << "2" << endl;
        }else{
            cout << "-1" << endl;
        }
    }
    return 0;
}