a = input()
l = int(input())
s = input()
co = []

def f (n, s):
    if n > 1:
        for i in a:
            f(n-1, s+i)
    else :
        co.append(s)

if l > len(a):
    m = l//len(a) +1
    a *= m
    a = a[:l]

f(l + 1, '')
for i in co:
    if i not in s:
        print(i)
        break