#include<bits/stdc++.h>
using namespace std;

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