using System;

public class Hello
{
    public static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var sa = line[0];
        var pa = line[1];
        line = Console.ReadLine().Trim().Split(' ');
        var sb = line[0];
        var pb = line[1];
        if (pa == pb) Console.WriteLine(-1);
        else Console.WriteLine(compAB(pa, pb) ? sa : sb);
    }
    public static bool compAB(string a, string b)
    {
        var aL = a.Length;
        var bL = b.Length;
        if (aL > bL) return true;
        else if (aL < bL) return false;
        var p = 0;
        while (true)
        {
            if (a[p] > b[p]) return true;
            else if (a[p] < b[p]) return false;
            p++;
        }
    }
}