def even_chars(st):
    """
    Finds all the even characters in a string.
    :param st: string value.
    :return: a sequence (index begins with 1) of all the even characters from a string. If the string is smaller than
             two characters or longer than 100 characters, the function should return "invalid string".
    """
    if len(st) < 2 or len(st) > 100: return "invalid string"
    else: return [st[i] for i in range(len(st)) if i % 2]