#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>

using namespace std;

int main() {
	int n, x, y;
	int ans;
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> x >> y;

		if (i == 0) {
			ans = y - x;
			if (ans <= 0) {
				cout << -1 << endl;
				return 0;
			}
		}

		if (ans != y - x) {
			cout << -1 << endl;
			return 0;
		}
	}

	cout << ans << endl;
	return 0;
}