#include "bits/stdc++.h"

#define rep(a,b) for(int a = 0;a < b;a++)
#define REP(i, x, n) for(int i = x; i < n; i++)
#define P(a) cout << a << endl
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
int dx[] = { 1, -1 , 0 , 0 };
int dy[] = { 0,  0,  1, -1 };
unsigned long long str_to_int(std::string str) {
	unsigned long long ret;
	std::stringstream ss; ss << str;
	ss >> ret;
	return ret;
}

int main() {
	int n, m;
	cin >> n >> m;
	if (n <= m) {
		P(1);
	}
	else {
		if (n % 2 == 1) {
			P(-1);
		}
		else {
			if (n / 2 > m) {
				P(-1);
			}
			else {
				P(2);
			}
		}
	}
}