def run_length_decoding(compressed_seq: str) -> str:
    seq = ""
    current_letter = None
    for character in compressed_seq:
        if character.isalpha():
            if current_letter is not None:
                seq += current_letter * int(number)

            current_letter = character
            number = ""
        else:
            # We assume that this is the number following the letter.
            number += character

    if current_letter is not None:
        seq += current_letter * int(number)

    return seq
    
print(run_length_decoding("A2B3C11"))