#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define SZ(n) (int)(n).size()
#define ALL(n) (n).begin(), (n).end()
#define MOD % 1000000007
using namespace std;
typedef long long LL;
typedef vector<int> VI;


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