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

class Program
{
    static int[] dx = new int[] { 0, 0, 1, -1 };
    static int[] dy = new int[] { 1, -1, 0, 0 };

    static void Solve()
    {
        var n = sc.Int();

        var x = sc.Int();
        var y = sc.Int();

        var ans = y - x;

        for (int i = 1; i < n; i++) {
            x = sc.Int();
            y = sc.Int();

            if (y - x != ans)
            {
                ans = -1;
                break;
            }
        }

        if (ans <= 0)
            ans = -1;

        pr.WriteLine(ans);
    }

    static int[,] Mul(int[,] a, int[,] b)
    {
        const int d = 2;
        var ret = new int[d, d];
        for (int i = 0; i < d; i++)
        {
            for (int j = 0; j < d; j++)
            {
                for (int k = 0; k < d; k++)
                {
                    ret[i, j] += a[i, k] * a[k, j];
                }
            }
        }

        return ret;
    }

    class Coord
    {
        public int x;
        public int y;
    }

    static void Main(string[] args)
    {
        // pr.AutoFlush = true;
        Solve();
        pr.Flush();
    }

    static Scanner sc = new Scanner();
    static Printer pr = new Printer();
}

#region IO
class Scanner
{
    private int _i = 0;
    private string[] line = new string[0];

    private T[] Enumerate<T>(int n, Func<T> f)
    {
        T[] ret = new T[n];
        for (int i = 0; i < n; i++)
            ret[i] = f();
        return ret;
    }

    public string Str()
    {
        if (line.Length <= _i)
        {
            line = Console.ReadLine().Split(' ');
            _i = 0;
        }

        return line[_i++];
    }

    public int Int() => int.Parse(Str());
    public long Long() => long.Parse(Str());
    public double Double() => double.Parse(Str());
    public int[] Int(int n) => Enumerate(n, Int);
    public long[] Long(int n) => Enumerate(n, Long);
    public double[] Double(int n) => Enumerate(n, Double);
    public string[] Str(int n) => Enumerate(n, Str);
}

class Printer : StreamWriter
{
    public Printer() : base(Console.OpenStandardOutput())
    {
        AutoFlush = false;
    }
}
#endregion