#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define repb(i, a, b) for(int i = a; i >= b; i--)
#define all(a) a.begin(), a.end()
#define o(a) cout << a << endl
#define int long long
#define fi first
#define se second
using namespace std;
typedef pair<int, int> P;

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