Lexicographically Minimal Subsequence

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

A 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 $s$.

A string $x = x_{1}x_{2}\dots x_{k}$ is lexicographically less than a string $y = y_{1}y_{2}\dots y_{k}$ if there exists a number $i$ $(1 \le i \le k)$ such 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 by their ASCII codes.