def minutestr(m):
    """
    Convert an integer to a 'HH:MM'-format string.

    >>> minutestr(70)
    '01:10'
    >>> minutestr(200.0)
    '03:20'
    >>> minutestr(180 * 60 + 30)
    '180:30'

    """
    return '{0:02.0f}:{1:02.0f}'.format(m // 60, m % 60)