def solve(a: str, b: str):
    if len(a) != len(b):
        return b if len(b) > len(a) else a
    else:
        for (x, y) in zip(map(int, a), map(int, b)):
            if x == 4 and y == 7 or (not x == 7 and y == 4) and x > y:
                return a
            elif x == 7 and y == 4 or x < y:
                return b


a, b = input().split()

print(solve(a, b))