#include <iostream>

using namespace std;

int solve(int n, int m) {
	if (n <= m) {
		return 1;
	}
	
	if (n % 2 != 0) {
		return -1;
	}
	
	if (n / 2 <= m) {
		return 2;
	}
	
	return -1;
}

int main () {
	
	int n, m;
	cin >> n >> m;
	
	int ret = solve(n, m);
	cout << ret << endl;
	
}