#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize ("Ofast")
#define F first
#define S second
#define vi vector<int>
#define vvi vector<vi>
#define pi pair<int, int>
#define vpi vector<pi>
#define vb vector<bool>
#define vvb vector<vb>
#define pb push_back
#define ppb pop_back
#define read(a) for(auto &x:a) cin >> x;
#define print(a) for(auto x:a) cout << x << " "; cout << "\n";
#define vc vector<char>
#define vvc vector<vc>
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define int long long
#define ld long double
const int INF = 4e18;
void solve(){
int n; cin >> n;
cout << (n%4==0 ? n/2 : -1) << "\n";
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
// #ifndef ONLINE_JUDGE
// freopen("in.txt", "r", stdin);
// freopen("out.txt", "w", stdout);
// #endif
int tt = 1;
// cin >> tt;
while(tt--)
solve();
return 0;
}
