#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

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

    int ans = 0;

    if(n >= m){
        ans = 1;
    }else if(n % 2 == 1){
        ans = -1;
    }else if(n/2 <= m){
        ans = 5;
    }else{
        ans = -1;
    }

    cout << ans << endl;
    return 0;
}