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

class Program
{
    public void Solve()
    {
        string[] ASPX = Console.ReadLine().Split(' ');
        string[] BSPX = Console.ReadLine().Split(' ');

        string ap = ASPX[1];
        string bp = BSPX[1];

        if (ap == bp) { Console.WriteLine(-1); }
        if (ap.Length != bp.Length)
        {
            if (ap.Length > bp.Length) { Console.WriteLine(ASPX[0]); return; }
            else { Console.WriteLine(BSPX[0]); return; }
        }
        for (int i = 0; i < ap.Length; i++)
        {
            if (int.Parse(ap.Substring(i, 1)) == int.Parse(bp.Substring(i, 1))) { continue; }

            if (int.Parse(ap.Substring(i, 1)) > int.Parse(bp.Substring(i, 1))) { Console.WriteLine(ASPX[0]); break; }
            else { Console.WriteLine(BSPX[0]); break; }
        }
    }

    static void Main()
    {
        var solver = new Program();
        solver.Solve();
    }
}