#include<bits/stdc++.h>
using namespace std;

int main(){
    int a;
    cin >> a;
    if(a <= 7){
        cout << -1 << endl;
        return 0;
    }
    int ans;
    for(int i = 8;;++i){
        if(a % i != 7) continue;
        if(i * 1 < a && i * 2 > a){
            ans = i;
            break;
        }
    }
    cout << ans << endl;
    return 0;
}