#!/usr/bin/python
import sys
import struct

class GetFPS(object):
    
    def __init__(self, filename):
	self.filename = filename

    def __enter__(self):
	return self.fps

    def fps(self):
        self.file = open(self.filename,"r+b")
        s = self.file.read(4)
	if s == "\x1a\x45\xdf\xa3":
	    return self.get_mkv_fps()
	elif s == "RIFF":
	    self.file.seek(32)
	    return 1000000.0 / float(struct.unpack('<I', self.file.read(4))[0])
	else:
	    raise Exception('Error: Unknown file format not AVI/MKV')

    def __exit__(self, type, value, traceback):
	try:
    	    self.file.close()
    	except:
    	    pass
        
    def eblm(self, bits = 0xf0):
        suma = 0x00
        mask = 0x01
        while not (suma & mask):
    	    suma =  ( suma << 8 ) + ord(self.file.read(1))
    	    if (mask == 0x01) and not (suma & bits):
		raise Exception('Error: MKV stream is broken')
	    mask <<= 7
	if bits == 0xf0:
	    return (suma, self.eblm( bits = 0xff ) )
	else:
	    return suma ^ mask

    def get_mkv_fps(self):
	track = 0
	self.file.seek(0)
	while 1:
	    class_id, length = self.eblm()
	    # print "class_id: %X length %i position:%i" % (class_id, length, self.file.tell())
	    if (class_id == 0x83):
		track = ord( self.file.read(1) )
	    elif (class_id == 0x23E383 and track == 1):
		break
	    elif (class_id not in [ 0x18538067, 0x1654AE6B, 0xAE, 0x83 ]):  #Segment,Tracks,TrackEntry,TrackType
		self.file.seek(length,1)

	return ( 1000000000/ float( struct.unpack('>I', self.file.read(4))[0] ) )


file = sys.argv[1]
try:
    with GetFPS(file) as _fps:
	fps = _fps()
except:
    print >> sys.stderr, "%s" %  sys.exc_info()[1]
    fps = 0
	
print "%.3f" % fps