D. Dense Subsequence time limit per test 2 seconds memory limit per test 256 megabytes input standard input output standard output

You are given a string s, consisting of lowercase English letters, and the integer m.

One should choose some symbols from the given string so that any contiguous subsegment of length m has at least one selected symbol. Note that here we choose positions of symbols, not the symbols themselves.

Then one uses the chosen symbols to form a new string. All symbols from the chosen position should be used, but we are allowed to rearrange them in any order.

Formally, we choose a subsequence of indices 1?≤?i1?<?i2?<?…?<?it?≤?|s|. The selected sequence must meet the following condition: for every j such that 1?≤?j?≤?|s|?-?m?+?1, there must be at least one selected index that belongs to the segment [j,? j?+?m?-?1], i.e. there should exist a k from 1 to t, such that j?≤?ik?≤?j?+?m?-?1.

Then we take any permutation p of the selected indices and form a new string sip1sip2… sipt.

Find the lexicographically smallest string, that can be obtained using this PRocedure. Input

The first line of the input contains a single integer m (1?≤?m?≤?100?000).

The second line contains the string s consisting of lowercase English letters. It is guaranteed that this string is non-empty and its length doesn’t exceed 100?000. It is also guaranteed that the number m doesn’t exceed the length of the string s. Output

Print the single line containing the lexicographically smallest string, that can be obtained using the procedure described above. Examples Input

3 cbabc

Output

a

Input

2 abcab

Output

aab

Input

3 bcabcbaccba

Output

aaabb

Note

In the first sample, one can choose the subsequence {3} and form a string “a”.

In the second sample, one can choose the subsequence {1,?2,?4} (symbols on this positions are ‘a’, ‘b’ and ‘a’) and rearrange the chosen symbols to form a string “aab”.

貪心，在[l + 1, l +ｍ－１]里選出最小，字典序最下，把比選中的最大的字符小的都加上？

AC代碼：