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

class Program
{
    static void Main(string[] args)
    {
        int n = int.Parse(Console.ReadLine());
        int output = -1;
        bool b = true;
        int m = 0;

        for (int i = 0; i < n; i++)
        {
            string[] s = Console.ReadLine().Split();
            int x = int.Parse(s[0]);
            int y = int.Parse(s[1]);

            if (n == 1)
            {
                b = false;
                break;
            }

            if (i == 0)
            {
                m = y - x;
            }
            else if (m != y - x)
            {
                b = false;
                break;
            }
        }

        if (b == true)
        {
            output = m;
        }

        Console.WriteLine(output);
        Console.ReadLine();
    }
}