#include <bits/stdc++.h>
using namespace std;

int main() {
	int N;
	cin >> N;

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

		int r = y - x;
		if (!i) {
			d = r;
		}
		else {
			if (d != r) {
				cout << -1 << endl;
				return 0;
			}
		}
	}

	cout << d << endl;

}