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 = "from collections import defaultdict\n\nclass DenseSubsequence():\n    def __init__(self, m, s):\n        self.m = m\n        self.s = s\n        self.chars = sorted(set(s))\n\n    def get_min_sequence(self, ch, mask):\n        lv, lch, i = -1, -1, 0\n        is_possible = True\n        seq = []\n        while i < len(self.s):\n            if mask[i] == 1:\n                lv = i\n            elif self.s[i] == ch:\n                lch = i\n\n            if i-lv == self.m:\n                if lch > lv:\n                    seq.append(lch)\n                    lv = lch\n                else:\n                    is_possible = False\n                    break\n            i += 1\n        if not is_possible:\n            return False, []\n        else:\n            return True, seq\n\n    def get_sequence(self):\n        char_map = defaultdict(list)\n        for i in range(len(self.s)):\n            char_map[self.s[i]].append(i)\n\n        mask = [0]*len(self.s)\n\n        res = ''\n\n        for ch in self.chars:\n            is_possible, seq = self.get_min_sequence(ch, mask)\n            if is_possible:\n                res = res+''.join([ch]*len(seq))\n                break\n            else:\n                res = res+''.join([ch]*len(char_map[ch]))\n                for v in char_map[ch]:\n                    mask[v] = 1\n        print(res)\n\nm = int(input())\ns = input().strip(' ')\nDenseSubsequence(m,s).get_sequence()"
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

```

Measured memory_bytes: 16591