using System;

class Program
{
    static void Main()
    {
        int N = int.Parse(Console.ReadLine());
        int? answer = null;

        for (int i = 0; i < N; i++)
        {
            var parts = Array.ConvertAll(Console.ReadLine().Split(), int.Parse);
            int a = parts[0], b = parts[1];
            int x = b - a;

            if (x <= 0)
            {
                answer = -1;
                break;
            }

            if (answer == null)
                answer = x;
            else if (answer != x)
            {
                answer = -1;
                break;
            }
        }

        Console.WriteLine(answer);
    }
}