#include <algorithm>
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(), (x).end()
#define ll long long
#define ld long double
#define INF 1000000000000000000
typedef pair<ll, ll> pll;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

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