#!/usr/bin/python2
from sys import argv
import os

NUMOFBITS = 4

def getSize(filename):
    st = os.stat(filename)
    return st.st_size

def usage():
	print "usage: ",argv[0],"<nbits> <inputfile> <outputfile>\n \tnbits should be a power of 2"

def checkif(sizefile,sizeimg,bitn):
	if sizeMax(sizeimg,bitn) > sizefile:
		return True
	return False

def sizeMax(sizefile,bitn):
	return (sizefile-54)*bitn/8 

def bits(bytes):
    r=[]
    for b in bytes:
	b = ord(b)
        for i in xrange(8):
            r.append((b >> i) & 1)
    return r[::-1]
def byty(bits):
	r = 0
	for b in bits:
		r = r << 1
		r = r + b
	return chr(r)

def bxor(b,c):
	b = ord(b)
	i = c[0]*2+c[1]
	return chr(b^i)

if len(argv)<3:
	usage()
	exit(-1)
NUMOFBITS = int(argv[1])
inf = open(argv[2],"r")
outf = open(argv[3],"w")
header = inf.read(54)
c = inf.read(8/int(NUMOFBITS))
while c:
	b=[]
#	b = bits(c[0])[6:8]+bits(c[1])[6:8]+bits(c[2])[6:8]+bits(c[3])[6:8]
	for d in c:
		b = b+ bits(d)[-NUMOFBITS:]
	outf.write(byty(b))
	c = inf.read(8/int(NUMOFBITS))