A, B = input().split()

def get_large(a, b):
    if len(a) != len(b):
        return (max(int(a), int(b)))
    else:
        for i in range(len(a)):
            if a[i] == b [i]:
                continue
            elif (a[i] == '4' and b[i] == '7') or (a[i] == '7' and b[i] == '4'):
                if a[i] == '4':
                    return a
                else:
                    return b
            else:
                return (max(int(a), int(b)))

print(get_large(A, B))