#include <iostream>
using namespace std;

int main(){
	int n, m;
	cin >> n >> m;
	if(n <= m) cout << 1 << endl;
	else if(m == 2 && (n == 2 || n == 4)) cout << n / 2 + 1 << endl;
	else if(n % 2 == 0 && n / 2 <= m) cout << 2 << endl;
	else cout << -1 << endl;
}