#include <iostream>
using namespace std;
int main(void){
    
    
    int n,m;
    cin >> n >> m;
    
    if(n >= m){
        cout << 1 << endl;
    }else if( n % 2 ){
        cout << -1 << endl;
    }else if(n / 2 <= m){
        cout << 2 << endl;
    }else{
        cout << -1 << endl;
    }
    
    
}