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>(N-1);

        int[] input = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
        N--;
        int ans = input[1] - input[0];

        var tmp = 0;
        while (N > 0)
        {
            input = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            tmp = input[1] - input[0];
            
            if (tmp <= 0) { Console.WriteLine(-1); return; }
            else if(tmp == ans) list.Add(ans);
            N--;
        }
        int count = list.Distinct().Count();
        Console.WriteLine(count == 0 ? -1 : list[0]);
    }
}