#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#include<stack>
#include<queue>
#include<list>
#include<bitset>
#include<functional>

#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007
#define INF 1000000000

using namespace std;

int main(void) {
	int n, m;

	cin >> n >> m;

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

	return 0;
}