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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def interval_to_milliseconds(interval):
 """Convert a Binance interval string to milliseconds :param interval: Binance interval string 1m, 3m, 5m, 15m, 30m, 1h, 2h, 4h, 6h, 8h, 12h, 1d, 3d, 1w :type interval: str :return: None if unit not one of m, h, d or w None if string not in correct format int value of interval in milliseconds """

    ms = None
    seconds_per_unit = {
        "m": 60,
        "h": 60 * 60,
        "d": 24 * 60 * 60,
        "w": 7 * 24 * 60 * 60
    }

    unit = interval[-1]
    if unit in seconds_per_unit:
        try:
            ms = int(interval[:-1]) * seconds_per_unit[unit] * 1000
        except ValueError:
            pass
    return ms