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

class Program
{
    public void Solve()
    {
        int N = int.Parse(Console.ReadLine());

        string[] XY = Console.ReadLine().Split(' ');
        int shikaku = int.Parse(XY[1]) - int.Parse(XY[0]);
        for (int i = 1; i < N; i++)
        {
            XY = Console.ReadLine().Split(' ');
            if (shikaku != int.Parse(XY[1]) - int.Parse(XY[0]))
            {
                shikaku = -1;
                break;
            }
        }
        if (shikaku <= 0) { shikaku = -1; }
        Console.WriteLine(shikaku);
    }

    static void Main()
    {
        var solver = new Program();
        solver.Solve();
    }
}