#include <iostream>
using namespace std;

int main()
{
	int N, x, y, sub;
	cin >> N >> x >> y;
	if ((sub = y - x) <= 0) {
		cout << -1 << endl;
		return 0;
	}
	for (int i = 1; i < N; i++) {
		cin >> x >> y;
		if (sub != y - x) {
			cout << -1 << endl;
			return 0;
		}
	}
	cout << sub << endl;
	return 0;
}