#include <stdio.h>
#include <algorithm>
#include <vector>
#include <deque>
#include <string>
#include <iostream>
using namespace std;
using ll = long long;

void solve();
int main() {
	solve();
#ifdef DBG
	while (true);
#endif
}

//531
void solve() {
	int n, m;
	scanf("%d %d", &n, &m);
	if (n <= m)
		printf("1\n");
	else if (!(n & 1) && n <= m * 2)
		printf("2\n");
	else
		printf("-1\n");
}