def a1(row_index, column_index):
    """Get an A1 notation for a cell specified by its row index
    and column index"""
    ord_first = ord('A')
    ord_last = ord('Z')
    letters_count = ord_last - ord_first + 1
    level, letter_index = divmod(column_index, letters_count)
    letter = chr(ord_first + letter_index) * (1 + level)
    number = row_index + 1
    return '{letter}{number}'.format(letter=letter, number=number)