@staticmethod
def resolve_problem_id(id_in):
    """Codeforces problems are encoded with uppercase latin letters. This
        function handles conversion from:

            - lowercase latin letters,
            - numbers.

        >>> SiteCodeforces.resolve_problem_id('A')
        'A'

        >>> SiteCodeforces.resolve_problem_id('z')
        'Z'

        >>> SiteCodeforces.resolve_problem_id('3')
        'C'

        >>> SiteCodeforces.resolve_problem_id('.') is None
        True
        """
    if isinstance(id_in, str):
        if id_in.isalpha() and len(id_in) == 1:
            return id_in.upper()
        if id_in.isdigit():
            id_out = chr(ord('A') + int(id_in) - 1)
            if id_out.isalpha():
                return id_out
    return None
