#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <numeric>
#include <bitset>
#include <complex>
#define rep(x, to) for (int x = 0; x < (to); x++)
#define REP(x, a, to) for (int x = (a); x < (to); x++)
#define foreach(itr, x) for (typeof((x).begin()) itr = (x).begin(); itr != (x).end(); itr++)
#define EPS (1e-14)

using namespace std;

typedef long long ll;
typedef pair<int, int> PII;
typedef pair<long, long> PLL;
typedef complex<double> Complex;
typedef vector< vector<int> > Mat;

int n;
int nx, ny;

int main() {
	cin >> n;
	rep(i, n) {
		int x, y;
		cin >> x >> y;
		nx += x;
		ny += y;
	}
	if ((ny - nx) % n == 0 && (ny - nx)/n > 0) {
		cout << (ny - nx) / n << endl;
	} else {
		cout << -1 << endl;
	}
	return 0;
}