using System;

public class Program {
	public static void Main(string[] args) {
		int n = int.Parse(Console.ReadLine());
		int ans = 0;
		for (int i = 0; i < n; i++) {
			string[] s = Console.ReadLine().Split(' ');
			int diff = int.Parse(s[1]) - int.Parse(s[0]);
			if (diff <= 0 || ans > 0 && diff != ans) {
				ans = -1;
				break;
			}
			ans = diff;
		}
		Console.WriteLine(ans);
	}
}