#coding: utf-8
##yuki_293

s=raw_input().split()

la=list(s[0])
lb=list(s[1])

a=int("".join(la))
b=int("".join(lb))

if len(la)>len(lb):
 res="".join(la)
elif len(la)<len(lb):
 res="".join(lb)
else: #len(la)==len(lb)
 for i in xrange(len(la)):
  if (la[i]=='7' and lb[i]=='4') or (la[i]=='4' and lb[i]=='7'):
   la[i],lb[i]=lb[i],la[i]

at=int("".join(la))
bt=int("".join(lb))
if at>=bt:
 print a
else:
 print b