def prepare_input(dirty: str) -> str:

    dirty = "".join([c.upper() for c in dirty if c in string.ascii_letters])
    clean = ""

    if len(dirty) < 2:
        return dirty

    for i in range(len(dirty) - 1):
        clean += dirty[i]

        if dirty[i] == dirty[i + 1]:
            clean += "X"

    clean += dirty[-1]

    if len(clean) & 1:
        clean += "X"

    return clean