#include<iostream>
#include<cstdio>
#include<vector>
using namespace std;

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

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

		int temp = y - x;
		if (temp > 0)
		{
			if (i == 0)
			{
				ans = temp;
			}
			else if (ans != temp)
			{
				ans = -1;
				break;
			}
		}
		else
		{
			ans = -1;
			break;
		}
	}
	cout << ans << endl;

	return 0;
}