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 ≤ i_1 < i_2 < ... < i_{t} ≤ |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 ≤ i_{k} ≤ j + m - 1.

Then we take any permutation p of the selected indices and form a new string s_{i}_{p}_1s_{i}_{p}_2... s_{i}_{p}_{t}.

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".

```python

USER_FILENAME = "<user_code>"
import sys, json, builtins
import math, heapq, bisect, itertools, functools, collections, statistics, fractions, decimal, operator, array, random, re, string, os, pathlib
try:
    import resource as _res
except Exception:
    _res = None
from collections import Counter, defaultdict, deque, OrderedDict
from functools import lru_cache, reduce
from itertools import accumulate, permutations, combinations, product
from math import gcd, ceil, floor, sqrt
from decimal import Decimal
try:
    from typing import *
except Exception:
    pass
try:
    sys.setrecursionlimit(1_000_000)
except Exception:
    pass
try:
    import site as _site
    try:
        _sys_sites = _site.getsitepackages() if hasattr(_site, "getsitepackages") else []
        for _p in _sys_sites:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
    try:
        _usp = _site.getusersitepackages()
        if isinstance(_usp, str):
            _usp = [_usp]
        for _p in _usp:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
except Exception:
    pass

ns = {"__name__": "__main__"}
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = "m = int(input())\ns = input().strip()\n\nsa = [0] * len(s)\nfor i in range(len(s)):\n\tsa[i] = ord(s[i]) - ord('a')\n\nsa = [-1] + sa + [-1]\n\ndef check_value(sa, m, threshold):\n\tprev_ind = 0\n\tfor i in range(len(sa)):\n\t\tif sa[i] <= threshold:\n\t\t\tif i - prev_ind <= m:\n\t\t\t\tprev_ind = i\n\t\t\telse:\n\t\t\t\treturn False\n\treturn True\n\ndef get_indexes(sa, threshold):\n\tseq = [i for i in range(len(sa)) if sa[i] <= threshold]\n\t# seq = []\n\t# for i in range(len(sa)):\n\t# \tif sa[i] < threshold:\n\t# \t\tseq[i].append(sa[i], i)\n\treturn seq\n\ndef filter_indexes(sa, seq, el, m):\n\tnew_seq = [0]\n\tfor i in range(1, len(seq) - 1):\n\t\tif sa[seq[i]] != el or (sa[seq[i]] == el and seq[i+1] - new_seq[-1] > m):\n\t\t\tnew_seq.append(seq[i])\n\treturn new_seq[1:]\n\n\nthreshold = -1\nwhile (not check_value(sa, m, threshold)):\n\t# print(threshold, get_indexes(sa, threshold))\n\tthreshold += 1\n# print(threshold, get_indexes(sa, threshold), sa)\n\nseq = get_indexes(sa, threshold)\nseq = filter_indexes(sa, seq, threshold, m)\n\ns = ''.join(sorted([chr(ord('a') + sa[x]) for x in seq]))\nprint(s)"
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

```

Measured memory_bytes: 13215