#!/usr/bin/env python3
def lt(x, y):
    if x == '7' and y == '4':
        return True
    elif x == '4' and y == '7':
        return False
    else:
        return x < y
xs, ys = input().split()
if len(xs) < len(ys):
    ans = ys
elif len(xs) > len(ys):
    ans = xs
else:
    for x, y in zip(xs, ys):
        if lt(x, y):
            ans = ys
            break
        elif lt(y, x):
            ans = xs
            break
    else:
        assert xs == ys
        ans = xs
print(ans)