#define _USE_MATH_DEFINES
#include<iostream>
#include <math.h>
#include <climits>
#include <algorithm>
#include <vector>
#include <queue>
#include <string>
using namespace std;

/* sample input

*/
int n, m;

int main()
{
	cin >> n >> m;

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

	return 0;
}