#include <iostream>
using namespace std;

int main(){
    int n, m;
    cin >> n >> m;
    if(n == 2){
        if(m == 1) cout << 2 << endl;
        else cout << 1 << endl;
    }else if(m >= n) cout << 1 << endl;
    else if((n+1)/2 <= m && n > 3) cout << 2 << endl;
    else cout << -1 << endl;
    return 0;
}