using System;

namespace yukicoder
{
    class Program
    {
        static void Main(string[] args)
        {
            int a = int.Parse(Console.ReadLine());
            int count = 0;
            int ans = 0;
            for (int i = 0; i < a; i++)
            {
                string[] s = Console.ReadLine().Split(' ');
                int b = int.Parse(s[0]);
                int c = int.Parse(s[1]);
                
                if (ans == c - b&&c-b!=0)
                {
                    count++;
                }
                if (i == 0)
                {
                    ans = c - b;
                    if (ans>=1)
                    {
                        count++;
                    }
                }
            }
            if (count == a)
            {
                Console.WriteLine(ans);
            }
            else
            {
                Console.WriteLine("-1");
            }
        }
    }
}