#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main() {
	int N;
	long long ans = 0;
	cin >> N;
	vector<long long> a(N), b(N), wid(N);
	for (int i = 0; i < N; i++) {
		cin >> a[i] >> b[i];
		wid[i] = a[i] + b[i] * 4;
	}
	sort(wid.begin(), wid.end());
	for (int i = 0; i < N - 1;i++) {
		long long temp = wid[N - 1] - wid[i];
		if (temp % 2 == 0) {
			ans += temp / 2;
		}
		else {
			cout << -1 << endl; return 0;
		}
	}
	cout << ans << endl;
	return 0;
}