from sys import stdin

def main():
    A, B = input().split()

    if len(A) == len(B):
        a, b = '', ''
        for s in zip(A, B):
            if '4' in s and '7' in s:
                s = [s[1], s[0]]
            a += s[0]
            b += s[1]
        a, b = int(a), int(b)
    else:
        a, b = int(A), int(B)
    print(max(a, b))

input = lambda: stdin.readline().rstrip()
main()