def METHOD_NAME(filepath: str) -> int:
    """
    Usage: C{get_png_dim(filepath:str) -> (int)}
    Finds the dimension of a PNG.
    @param filepath: file path of the PNG.
    @returns: (width, height).
    @raise Exception: Raised if the file is not a png
    """
    if not imghdr.what(filepath) == 'png':
        raise Exception("not PNG")
    head = open(filepath, 'rb').read(24)
    return struct.unpack('!II', head[16:24])