#include <iostream>

using namespace std;

int main()
{

	int N, X, Y;
	cin >> N;

	int ans;
	cin >> X >> Y;
	ans = Y - X;
	if( ans <= 0 ) {
		cout << -1 << endl;
		return 0;
	}
	for( int i = 1; i < N; i++ ) {
		cin >> X >> Y;
		if( Y - X != ans ) {
			cout << -1 << endl;
			return 0;
		}
	}
	cout << ans << endl;

	return 0;
}