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

class Program
{
    static void Main(string[] args)
    {
        var N = int.Parse(Console.ReadLine());
        var set = new HashSet<int>();

        int[] input;
        int ans = 0;
        while (N > 0)
        {
            input = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            input[1] -= input[0];
            if (input[1] <= 0) { Console.WriteLine(-1); return; }
            if (ans == 0) ans = input[1];
            else if (ans != input[1]) { Console.WriteLine("-1"); return; }
        }
        Console.WriteLine(ans);
    }
}