a, b = list(map(int, input().split(' ')))

def compare(a, b):
    def _compare(al, bl):
        if len(al) < len(bl):
            return True
        elif len(al) > len(bl) or (len(al) == 0 and len(bl) == 0):
            return False
        else:
            if al[0] == 4 and bl[0] == 7:
                return False
            elif al[0] == 7 and bl[0] == 4:
                return True
            elif al[0] != bl[0]:
                return al[0] < bl[0]
            else:
                return _compare(al[1:], bl[1:])
            
    al = list(map(int, list(str(a))))
    bl = list(map(int, list(str(b))))
    return _compare(al, bl)


if compare(a, b):
    print(b)
else:
    print(a)