from sys import stdin

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

    def func(s):
        ret = ''
        for x in s:
            if x == '4':
                ret += '7'
            elif x == '7':
                ret += '4'
            else:
                ret += x
        return int(ret)

    if ('4' in A and '7' in B) or ('7' in A and '4' in B):
        a, b = func(A), func(B)
    else:
        a, b = int(A), int(B)

    if a > b:
        print(A)
    else:
        print(B)

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