s, t = raw_input().split()

def compare(a, b):
  for i in xrange(len(a)):
    if a[i] == "4" and b[i] == "7":
      return a
    elif a[i] == "7" and b[i] == "4":
      return b
    elif a[i] > b[i]:
      return a
    elif a[i] < b[i]:
      return b
  return a

if len(s) > len(t):
  print s
elif len(s) < len(t):
  print t
else:
  print compare(s, t)