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

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