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

namespace yukicoderTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            int N = int.Parse(input);
            int[] ans = new int[N];

            for (int i = 0; i < N; i++)
            {
                string input2 = Console.ReadLine();
                string[] lr = input2.Split(' ');
                int left = int.Parse(lr[0]);
                int right = int.Parse(lr[1]);
                ans[i] = (left - right) * -1;
            }
            int write = ans[0];
            for (int i = 1; i < N; i++)
            {
                if (write < 0 || write != ans[i])
                {
                    write = -1;
                    break;
                }

            }

            if (write < 0)
            {
                Console.WriteLine("-1");
            }
            else
            {
                Console.WriteLine(write);
            }
        
        }

    }
}