#!/usr/bin/env python

import sys

hexchars = "0123456789abcdef"
oldindex = None

while True:
    x = sys.stdin.read(1)
    if not x:
        break

    x = x.lower()

    if x not in hexchars:
        continue

    i = hexchars.index(x)

    if oldindex is None:
        oldindex = i
    else:
        n = (oldindex << 4) | i
        sys.stdout.write("%c" % n)
        oldindex = None