#nullable enable

#region
var (_input, _iter) = (Array.Empty<string>(), 0);
T I<T>() where T : IParsable<T>
{
    while (_iter >= _input.Length) (_input, _iter) = (Console.ReadLine()!.Trim().Split(' '), 0);
    return T.Parse(_input[_iter++], null);
}
#endregion

var s = I<string>();
var e = I<string>();
var n = I<int>();
if (n == 1)
{
    if (s == e)
    {
        Console.WriteLine(s);
        return;
    }
    Console.WriteLine(-1);
    return;
}
var ans = new List<char>(){ s[0] };
while (ans.Count < n - 1) ans.Add('a');
ans.Add(e[0]);
Console.WriteLine(new string([..ans]));