using System;

public class Hello
{
    public static void Main()
    {
        var n = int.Parse(Console.ReadLine().Trim());
        string[] line = Console.ReadLine().Trim().Split(' ');
        var x = int.Parse(line[0]);
        var y = int.Parse(line[1]);
        int ans;
        if (x >= y) ans = -1;
        else ans = y - x;
        for (int i = 1; i < n; i++)
        {
             line = Console.ReadLine().Trim().Split(' ');
            if (ans == -1) continue;
             x = int.Parse(line[0]);
             y = int.Parse(line[1]);
            if (x >= y) { ans = -1; continue; }
            if (y -x != ans) { ans = -1; continue; }
        }
        Console.WriteLine(ans);
    }
}