using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Programming
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string[] str;
            int[] X = new int[N];
            int[] Y = new int[N];
            for (int i = 0; i < N; i++)
            {
                str = Console.ReadLine().Split();
                X[i] = int.Parse(str[0]);
                Y[i] = int.Parse(str[1]);
            }

            //-----------------------------

            int ans = Y[0] - X[0];
            for (int i = 1; i < N ; i++)
            {
                if (ans != Y[i] - X[i])
                {
                    ans = -1;
                    Console.WriteLine(ans);
                    return;
                }
            }

            if (ans <= 0)
            {
                ans = -1;
            }

            Console.WriteLine(ans);
        }
    }
}