#include<iostream>
using namespace std;
int main(void)
{
	int n, i, x, y, ans, p, ch = 0;

	cin >> n;
	for (i = 0; i < n; i++) {
		cin >> x >> y;
		p = y - x;
		if (p > 0) {
			ch+=1;
				if (i == 0) 
					ans = p;	
				else if (ans != p)ch = 0;
		}
		else { ch = 0; 
		}
	}

	if (ch == n)
		cout << ans << endl;
	else { cout << -1 << endl; }

	return 0;
}