using System;
using System.Text;
using System.Linq;

namespace ConsoleApp11
{
    class Program
    {
        static void Main(string[] args)
        {
            long N = long.Parse(Console.ReadLine());

            long[] X = new long[N];
            long[] Y = new long[N];

            int tof = -1;

            for(long i = 0; i < N; i++)
            {
                var input = Console.ReadLine().Trim().Split(' ');

                X[i] = long.Parse(input[0]);
                Y[i] = long.Parse(input[1]);
            }

            for(long i = 0; i < N - 1; i++)
            {
                if(N > 1)
                {
                    if ((Y[i] - X[i]) == Y[i + 1] - X[i + 1] && (Y[i] - X[i]) >= 0)
                    {
                        tof = 1;
                    }
                    else
                    {
                        tof = -1;
                        break;
                    }
                }
                else
                {
                    if(Y[i] - X[i] >= 0)
                    {
                        tof = 1;
                    }
                    else
                    {
                        tof = -1;
                    }
                }
            }

            if(tof == 1)
            {
                Console.WriteLine(Y[0] - X[0]);
            }
            else if(tof == -1)
            {
                Console.WriteLine(tof);
            }
        }        
    }
}