#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;
#ifdef _WIN32
#include <Windows.h>
#endif

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