using System;
using System.IO;

class Program
{
    static void Main()
    {
        using var sr = new StreamReader(Console.OpenStandardInput(), bufferSize: 1 << 16);
        using var sw = new StreamWriter(Console.OpenStandardOutput());
        // 1行目
        string? s = sr.ReadLine();
        int count = int.Parse(s!);

        int ansSave = int.MinValue; // 未設定を表す
        for (int i = 0; i < count; i++)
        {
            string? line = sr.ReadLine();
            if (line is null) { sw.WriteLine(-1); return; }

            int sp = line.IndexOf(' ');
            if (sp < 0) { sw.WriteLine(-1); return; }

            ReadOnlySpan<char> a = line.AsSpan(0, sp);
            ReadOnlySpan<char> b = line.AsSpan(sp + 1);

            if (!int.TryParse(a, out int left) || !int.TryParse(b, out int right))
            {
                sw.WriteLine(-1);
                return;
            }

            int diff = right - left;
            if (i == 0)
            {
                ansSave = diff;
            }
            else if (ansSave != diff)
            {
                sw.WriteLine(-1);
                return; // 早期終了
            }
        }

        if (ansSave != int.MinValue)
            sw.WriteLine(ansSave);
    }
}