主打一个暴力美学

s = input()
r = int(input())
n = len(s)
r = n-r
ans = [0 for i in range(n+10)]
vis = [True for i in range(n+10)]
t = []

def dfs(k):
    if k>r:
        t.append(ans[1:r+1])
        return 
    for i in range(ans[k-1]+1,n+1):
        if vis[i]:
            ans[k] = i
            vis[i] = False
            dfs(k+1)
            vis[i] = True
dfs(1)

change = []
for i in t:
    s1 = ''
    for j in i:
        s1+=s[j-1]
    change.append(s1)

change.sort()
print(change[0])