#include<iostream>
#include<string>
#include<cmath>
#include<set>
using namespace std;
int main(void){
    int n,m;
    cin >> n >> m;
    if(n<=m)    cout << 1 << endl;
    else if(n/2<=m&&n%2==0)  cout << 2 << endl;
    else    cout << -1 << endl;
    return 0;
}