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());
            int[,] a = new int[N, 2];
            string[] str;

            for (int i = 0; i < N; i++)
            {
                str = Console.ReadLine().Split();
                a[i, 0] = int.Parse(str[0]);
                a[i, 1] = int.Parse(str[1]);
            }

            int ans;

            ans = a[0, 1] - a[0, 0];
            if (ans < 0)
            {
                ans = -1;
                Console.WriteLine(ans);
                return;
            }

            for (int i = 0; i < N; i++)
            {
                if (ans != a[i, 1] - a[i, 0])
                {
                    ans = -1;
                    break;
                }
            }

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