#include <iostream>

using namespace std;

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

	int ans = -1;
	if (m >= n) {
		ans = 1;
	} else if (n == 3) {
		ans = -1;
	} else if (m >= ((n + 1) / 2)) {
		ans = 2;
	}

	cout << ans << endl;
	return 0;
}