#include <bits/stdc++.h>
using namespace std;
#define int long long
const int INF = 1e18;
const int MOD = 1e9+7;
const int MAXN = 5000;
int n;
void mulaidarinol(){}
void solve(){
cin >> n;
for(int i = 1; i <= n; i++){
int u, v; cin >> u >> v;
}
if(n % 4 == 0) cout << n / 2 << endl;
else cout << -1 << endl;
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int tc = 1;
// cin >> tc;
while(tc--){
// mulaidarinol();
solve();
// cerr << endl;
}
return 0;
}
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