def decode(s):
    """
    Run length decoding
    (str) -> str
    
    >>> decode('1B5W1B4W')
    'BWWWWWBWWWW'
    """
    ret = ''
    sizeStr = ''
    for ch in s:
        if ch.isalpha():
            if sizeStr:
                ret += ch * int(sizeStr)
                sizeStr = ''
        else:
            sizeStr += ch
    
    return ret