#include<iostream>
using namespace std;

int main() {
	int n, ans;
	cin >> n;
	for (int i = 0;i < n;i++) {
		int a, b;
		cin >> a >> b;
		if (i == 0)ans = b - a;
		else if (ans != b - a) {
			cout << -1 << endl;
			return 0;
		}
	}
	cout << (ans > 0 ? ans : -1) << endl;
}