#include <iostream>
#define rep(i, n) for (int i = 0; i < n; i++)

using namespace std;

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

	int ans;
	rep(i, n)
	{
		int x, y;
		cin >> x >> y;

		int z = y - x;
		ans = (z > 0 && (i == 0 || ans == z)) ? z : -1;
	}
	cout << ans << endl;
}