def convert(bytes_):
    """
    takes a number of bytes as an argument and returns the most suitable human
    readable unit conversion.
    """
    if bytes_ > 1024 ** 3:
        hr = round(bytes_ / 1024 ** 3)
        unit = 'GB'
    elif bytes_ > 1024 ** 2:
        hr = round(bytes_ / 1024 ** 2)
        unit = 'MB'
    else:
        hr = round(bytes_ / 1024)
        unit = 'KB'
    hr = str(hr)
    result = hr + ' ' + unit
    return result
