#include <bits/stdc++.h>
#define FOR(i,bg,ed) for(ll i=(bg);i<(ed);i++)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007
//#define int long long
using namespace std;
typedef long long ll;
typedef vector<int> V;
typedef vector<V> VV;
const int INF = 1e9;

signed main()
{
    int n, m;

    cin >> n >> m;
    if (n % 2 == 0) {
        if (n <= m) {
            cout << 1 << endl;
        } else {
            if (n / 2 <= m) {
                cout << 2 << endl;
            } else {
                cout << -1 << endl;
            }
        }
    } else {
        if (n == 3) {
            if (m >= 3) {
                cout << 1 << endl;
                return 0;
            } else {
                cout << -1 << endl;
                return 0;
            }
        }
        if (n <= m) {
            cout << 1 << endl;
        } else {
            if (n / 2 + 1 <= m) {
                cout << 2 << endl;
            } else {
                cout << -1 << endl;
            }
        }
    }
}