using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var N = int.Parse(Console.ReadLine());
        var list = new List<int>();

        int[] input;
        var result = 0;
        for (int i = 0; i < N; i++)
        {
            input = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            result = input[1] - input[0];

            if (result >= 0) list.Add(result);
        }
        if (list.Count() == 0) Console.WriteLine(-1);
        else Console.WriteLine((list.Distinct().Count() == 1) ? list[0] : -1);
    }
}