Lexicographically Minimal Subsequence

Your are given a string $s$ and an integer $k$. Find the lexicographically minimal subsequence of $s$ which length is $k$.

The first line contains a string $s$ ($1 \le |s| \le 10^6$). It consists of lowercase Latin letters.

The second line contains an integer $k$ ($1 \le k \le |s|$) --- the length of the resulting subsequence.

Output the lexicographically minimal subsequence of $s$ which length is $k$.

String $s\_{p\_{1}}s\_{p\_{2}}\dots s\_{p\_{k}} (1 \le p\_{1} < p\_{2} < \dots < p\_{k} \le |s|)$ is called a subsequence of string $s$.

String $x = x\_{1}x\_{2}\dots x\_{k}$ is lexicographically less than string $y = y\_{1}y\_{2}\dots y\_{k}$, if there exists such number $i$ $(1 \le i \le k)$, that $x\_{1} = y\_{1}, x\_{2} = y\_{2}, \ldots , x\_{i-1} = y\_{i-1}$ and $x\_{i} < y\_{i}$. Characters in strings are compared as their ASCII codes.