using System;

namespace yukicoder
{
    class Program
    {

        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());

            int[] strings1 = new int[N * 2];
            //int[] strings2 = new int[str.Length];

            for (int i = 0; i <= N * 2 - 1; i++)
            {
                var str = Console.ReadLine().Split(" ");
                int n = 0;
                strings1[i] = int.Parse(str[n]);
                strings1[i + 1] = int.Parse(str[n + 1]);
                i++;
            }
            

            for (int k = strings1.Length -1; k >= 0;)
            {
                if (strings1[k] - strings1[k - 1] == strings1[k - 2] - strings1[k - 3])
                {
                    k = k - 2;
                    if(k <= 1 && strings1[1] - strings1[0] > 0)
                    {
                        Console.WriteLine(strings1[1] - strings1[0]);
                        break;
                    }
                    else
                    {
                        Console.WriteLine("-1");
                        break;
                    }
                    continue;
                }
                else
                {
                    Console.WriteLine("-1");
                    break;
                }
                
            }
           
        }
    }
}