import sys
input=lambda: sys.stdin.readline().rstrip()
a,b=map(int,input().split())
aa,bb=str(a),str(b)

def f(x):
  x=x.replace("4","a")
  x=x.replace("7","4")
  x=x.replace("a","7")
  return x

aa=int(f(aa))
bb=int(f(bb))
if aa>bb:
  print(a)
else:
  print(b)