CF844A Diversity

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• 评测方式 RemoteJudge
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题意翻译

给出一个字符串$S$与一个整数$k$。

你可以更改$S$中的任意字符，使得字符串$S$中有不小于$k$个不同的字符。

求最少的更改次数。

$1\le k \le 26, 1\le |S|\le 1000.$

题目描述

Calculate the minimum number of characters you need to change in the string $s$ , so that it contains at least $k$ different letters, or print that it is impossible.

String $s$ consists only of lowercase Latin letters, and it is allowed to change characters only to lowercase Latin letters too.

输入输出格式

输入格式：

First line of input contains string $s$ , consisting only of lowercase Latin letters ( $1<=|s|<=1000$ , $|s|$ denotes the length of $s$ ).

Second line of input contains integer $k$ ( $1<=k<=26$ ).

输出格式：

Print single line with a minimum number of necessary changes, or the word «impossible» (without quotes) if it is impossible.

输入输出样例

输入样例#1： 复制
yandex
6

输出样例#1： 复制
0

输入样例#2： 复制
yahoo
5

输出样例#2： 复制
1

输入样例#3： 复制
google
7

输出样例#3： 复制
impossible


说明

In the first test case string contains $6$ different letters, so we don't need to change anything.

In the second test case string contains $4$ different letters: ${'a','h','o','y'}$ . To get $5$ different letters it is necessary to change one occurrence of $'o'$ to some letter, which doesn't occur in the string, for example, ${'b'}$ .

In the third test case, it is impossible to make $7$ different letters because the length of the string is $6$ .

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