#include<iostream>
using namespace std;
int main(void){
	int x, y, n;
	cin >> n >> x >> y;
	int ans = y - x;
	if (ans < 0) { cout << -1 << endl; return 0; }

	for (int i = 0; i < n-1; i++){
		cin >> x >> y;
		if (y - x != ans || y - x < 0) { cout << -1 << endl; return 0; }
	}
	cout << ans << endl;
	return 0;
}