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

class yc2
{
    static void Main()
    {
        int N = int.Parse(Console.ReadLine());
        string[] str1;
        int[,] X = new int[2,N];

        for(int i=0;i<N;i++)
        {
            str1 = Console.ReadLine().Split();
            X[0,i] = int.Parse(str1[0]);
            X[1,i] = int.Parse(str1[1]);
            if (X[1, i] - X[0, i] < 1)
            {
                Console.WriteLine(-1);
                return;
            }
            else if (i >= 1 && X[1, i] - X[0, i] != X[1, i - 1] - X[0, i - 1])
            {
                Console.WriteLine(-1);
                return;
            }
        }
        Console.WriteLine(X[1, 0] - X[0, 0]);
    }
}