#include<iostream>
typedef long long int ll;
using namespace std;
ll N,M;
int main(){
    ios::sync_with_stdio(false);
    cin>>N>>M;
    if (N <= M) cout << 1 << endl;
    else if (N % 2 == 0 && N <= M * 2)  cout << 2 << endl;
    else   cout << -1 << endl;
    return 0;
}