#!/usr/bin/python
def bye(x): print x; exit(0)

a, b = raw_input().split()
ha = max(len(a), len(b))
za = a.zfill(ha)
zb = b.zfill(ha)
for x, y in zip(za, zb):
    if x == y:
        continue
    if x == '4' and y == '7':
        bye(a)
    elif x == '7' and y == '4':
        bye(b)
    else:
        bye(a if x > y else b)