#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <stack>
#include <algorithm>
#include <string>
#include <map>
#include <iterator>
#include <set>
#include <queue>
#include <bitset>
#include <cassert>

using namespace std;
using ll = long long;


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