//Rahman ve Rahim olan Allah'ın adıyla
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define mid ((st+end)/2)
#define hes (pre[n]-k2-(pre[r+1]-pre[l]))
#define fi first
#define se second
const int INF=1e15+5;
const int E16=1e6;
const int ONE=1;
const int TWO=2;
const int ZERO=0;
const int MOD=1e9+7;//(x+MOD)%MOD
//el.erase(unique(el.begin(),el.end()),el.end());
int n,k;
int32_t main(){
ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
cin>>n;
if(n%2){cout<<-1;}
else{cout<<n/2;}
}
