A, B = input().split()
A1 = A.replace('4', 'X')
A1 = A1.replace('7', '4')
A1 = A1.replace('X', '7')
A1 = int(A1)
B1 = B.replace('4', 'X')
B1 = B1.replace('7', '4')
B1 = B1.replace('X', '7')
B1 = int(B1)
if A1 > B1:
    print(A)
else:
    print(B)