A,B = map(str,input().split())
A1 = A.replace('7','!')
A1 = A1.replace('4','?')
A1 = A1.replace('?','7')
A1 = A1.replace('!','4')
B1 = B.replace('7','!')
B1 = B1.replace('4','?')
B1 = B1.replace('?','7')
B1 = B1.replace('!','4')
if int(A1) < int(B1):
    print(B)
else:
    print(A)