#include <algorithm>
#include <array>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <optional>
#include <queue>
#include <sstream>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

int main() {
    int32_t n, m;
    cin >> n >> m;
    int32_t ans;
    if (n <= m)
        ans = 1;
    else if (n % 2 == 0 && n / 2 <= m)
        ans = 2;
    else
        ans = -1;
    cout << ans << endl;
    return 0;
}