#include <bits/stdc++.h>
using namespace std;

int main()
{
int n;
int m;

	cin >> n;
	cin >> m;

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

	return 0;
}