def f(s):
    a, b = s.split()
    la, lb = len(a), len(b)
    if la > lb: return a
    if la < lb: return b
    for i in range(la):
        if a[i] != b[i]:
            if a[i]=="4" and b[i]=="7": return a
            if a[i]=="7" and b[i]=="4": return b
            return a if a[i]>b[i] else b
print(f(input()))