#include <bits/stdc++.h>
#define PI 3.14159265359
#define NIL -1
using namespace std;
const int64_t MOD = 1e9 + 7;

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

	if (n <= m) {
		cout << 1 << endl;
		return 0;
	}
	else {
		if (n % 2) {
			cout << -1 << endl;
			return 0;
		}
		else {
			if ((n % 2 == 0) && (n / 2 == m)) {
				cout << 4 << endl;
				return 0;
			}
		}
	}

	cout << 1 << endl;
	return 0;
}