// written by Asedre
#include <iostream>
#include <cmath>
#include <climits>
#include <set>
#include <map>
#include <iomanip>
#include <vector>
#include <string>
#include <algorithm>
#include <utility>
#include <queue>
#include <numeric>
#include <stack>
#include <assert.h>
#include <unordered_set>
#include <cstring>
#include <unordered_map>
#define pb push_back
#define ll long long
#define ld long double
#define all(x) x.begin(), x.end()
#define allr(x) x.rbegin(), x.rend()
#define ull unsigned long long
using namespace std;
const int maxv = 1e6 + 3, maxn = 1e6 + 12, LOG = 25;
const int inf = 1e9 + 5;
const int MOD = 1e9 + 7, MOD2 = 1338519349;
const double PI = 3.14159265359;
//prime nums: 9059, 5419, 3571
void teztez( ) {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
}/*
ll binpow(ll x, ll p, ll mod) {
if (!p) return 1;
if (p % 2) return (x % mod * binpow(x, p - 1, mod)) % mod;
ll xx = binpow(x, p / 2, mod);
return (xx % mod * xx % mod) % mod;
}
*/
ll binpow(ll x, ll p) {
if (!p) return 1;
if (p % 2) return (x % MOD * binpow(x, p - 1) % MOD) % MOD;
ll xx = binpow(x, p / 2) % MOD;
return (xx * xx) % MOD;
}
ll gcd(ll a,ll b) {
return b ? gcd(b, a % b) : a;
}
ll lcm(ll a, ll b) {
return abs(a * b) / gcd(a, b);
}
pair <int, int> p[maxv];
void solve( ) {
int n; cin >> n;
for (int i = 1; i <= n; i ++) {
cin >> p[i].first >> p[i].second;
}
if (n % 4 == 0) cout << n / 2 << "\n";
else {
cout << -1 << "\n";
}
}
signed main( ) {
teztez( );
//freopen("angle2.in", "r", stdin); freopen("angle2.out", "w", stdout);
int testcases = 1;
//cin >> testcases;
while (testcases--) {
solve( );
}
return 0;
}
