#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
using namespace std;

int main()
{
    int n, m;
    cin >> n >> m;

    if(n <= m)
        cout << 1 << endl;
    else if(n % 2 == 0 && n / 2 <= m)
        cout << 2 << endl;
    else
        cout << -1 << endl;

    return 0;
}