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

class Program
{
    static string InputPattern = "Input4";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("2");
            WillReturn.Add("3 5");
            WillReturn.Add("7 9");
            //2
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("2");
            WillReturn.Add("3 5");
            WillReturn.Add("7 10");
            //-1
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("1");
            WillReturn.Add("8 2");
            //-1
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();

        var SikakuSet = new HashSet<int>();
        foreach (string EachStr in InputList.Skip(1)) {
            int[] wkArr = EachStr.Split(' ').Select(X => int.Parse(X)).ToArray();
            SikakuSet.Add(wkArr[1] - wkArr[0]);
        }

        if (SikakuSet.Count >= 2) { //ユニーク解でなければNG
            Console.WriteLine(-1); return;
        }
        if (SikakuSet.First() <= 0) { //正整数の解でなければNG
            Console.WriteLine(-1); return;
        }
        Console.WriteLine(SikakuSet.First());
    }
}