# Task

Christmas is coming. In the [previous kata](https://www.codewars.com/kata/5a405ba4e1ce0e1d7800012e), we build a custom Christmas tree with the specified characters and the specified height.

Now, we are interested in the center of the Christmas tree. 

Please imagine that we build a Christmas tree with `chars = "abc" and n = Infinity`, we got:
```
          a
         b c
        a b c
       a b c a
      b c a b c
     a b c a b c
    a b c a b c a
   b c a b c a b c
  a b c a b c a b a
 b c a b c a b a b c
a b c a b a b c a b c
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
          |
          |
          .
          .
```
If we keep only the center part of leaves, we will got:
```
          a
            
          b  
              
          a    
                
          a        
                    
          b        
                    
          a          
          .
          
          .
          
          .
```
As you can see, it's a infinite string, but it has a repeat substring "aba"(spaces will be removed). If we join them together, it looks like:`"abaabaabaabaaba......"`.

So, your task is to find the repeat substring of the center part of leaves.

# Inputs:

- `chars`: the specified characters. In this kata, they always be lowercase letters.

# Output:

- The repeat substring that satisfy the above conditions.

Still not understand the task? Look at the following example ;-)

# Examples


For `chars = "abc"`,the output should be `"aba"`
```
center leaves sequence: "abaabaabaabaabaaba....."
```
For `chars = "abab"`,the output should be `a`
```
center leaves sequence: "aaaaaaaaaa....."
```
For `chars = "abcde"`,the output should be `aecea`
```
center leaves sequence: "aeceaaeceaaecea....."
```
For `chars = "aaaaaaaaaaaaaa"`,the output should be `a`
```
center leaves sequence: "aaaaaaaaaaaaaa....."
```
For `chars = "abaabaaab"`,the output should be `aba`
```
center leaves sequence: "abaabaabaaba....."
```

```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

def _disabled_input(*args, **kwargs):
    raise RuntimeError("input() is disabled in callable mode")
builtins.input = _disabled_input

ns = {"__name__": "__judge__"}  # prevent if __name__ == "__main__" blocks
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = "def center_of(chars):\n    if chars == '':\n        return ''\n    chars = chars * 10000\n    def n_int(n):\n        return n*(n+1) // 2\n    center = ''\n    c = 0\n    i= 0\n    while n_int(i) <= len(chars):\n        if i%2 != 0:\n            c += 1\n            pos_center = (n_int(i) - c) \n            center += chars[pos_center]\n        i += 1\n    for x in range(1, len(center)):\n        subcenter = center[:x]\n        if subcenter * (len(center)//len(subcenter))+(subcenter[:len(center)%len(subcenter)]) == center:\n            return(subcenter)"
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'center_of'
if _fn_name:
    if "Solution" in ns:
        try:
            _obj = ns["Solution"]()
            if hasattr(_obj, _fn_name):
                _target = getattr(_obj, _fn_name)
        except Exception:
            _target = None
    if _target is None and _fn_name in ns and callable(ns[_fn_name]):
        _target = ns[_fn_name]
# Fallback: if there's exactly one user-defined function, use it
if _target is None:
    _cands = [v for k, v in ns.items() if callable(v) and getattr(v, "__module__", "") in ("__judge__", "__main__")]
    _user_funcs = [c for c in _cands if getattr(c, "__name__", "").startswith("_") is False]
    if len(_user_funcs) == 1:
        _target = _user_funcs[0]
if _target is None:
    raise RuntimeError(f"Could not resolve callable '{_fn_name}'")

_raw = sys.stdin.read().strip()
if _raw:
    _parsed = json.loads(_raw)
    if isinstance(_parsed, dict):
        _kwargs = _parsed.get("kwargs", {})
        _args = _parsed.get("args", [])
    else:
        _args, _kwargs = _parsed, {}
else:
    _args, _kwargs = [], {}
if not isinstance(_args, (list, tuple)):
    _args = [_args]
_res = _target(*_args, **_kwargs)
try:
    sys.stdout.write(repr(_res) + "\n")
except Exception:
    pass

```

Measured memory_bytes: 5758