#include<bits/stdc++.h>
#include <atcoder/all>
#ifdef LOCAL
#include <debug_print.hpp>
#define debug(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) (static_cast<void>(0))
#endif
using namespace atcoder;
using mint=modint998244353;
using namespace std;
using ll=long long;
using ul=unsigned long long;
int dx[8] = {-1, 1, 0, 0, -1, -1, 1, 1};
int dy[8] = {0, 0, -1, 1, -1, 1, -1, 1};
using Graph=vector<vector<int>>;
int op(ll a,ll b){return int(a+b);}
int e(){return 0;}
int main(){
    int A;
    cin>>A;
    for(int i=8;i<=100;i++)if(A%i==7&&i<=A&&A<i*2){cout<<i<<endl;return 0;}
    cout<<-1<<endl;
}