#include <iostream> 
using namespace std;

int main() {
    int 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;
    }
}