#if !defined(__clang__) && defined(__GNUC__)
#include <bits/stdc++.h>
#else
#include <cstdlib>
#include <climits>
#include <iostream>
#include <cstdint>
#include <vector>
#include <string>
#include <complex>
#include <bitset>
#include <queue>
#include <deque>
#include <stack>
#include <utility>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <regex>
#endif //  !defined(__clang__) && defined(__GNUG__)
#if 0
#include <boost/lexical_cast.hpp>
#include <boost/algorithm/string.hpp>
#include <boost/multi_array.hpp>
#include <boost/optional.hpp>
#include <boost/math/common_factor_rt.hpp>
#endif

using namespace std;

int main()
{
    int64_t N, M;
    std::cin >> N >> M;

    if (N <= M) {
        std::cout << 1 << std::endl;
    }
    else {
        if ((N % 2 == 0) && N / 2 <= M) {
            std::cout << 2 << std::endl;
        }
        else {
            std::cout << -1 << std::endl;
        }
    }


    return 0;
}