Description:

The mean (or average) is the most popular measure of central tendency; however it does not behave very well when the data is skewed (i.e. wages distribution). In such cases, it's better to use the median.

Your task for this kata is to find the median of an array consisting of n elements.

You can assume that all inputs are arrays of numbers in integer format. For the empty array your code should return `NaN` (false in JavaScript/`NULL` in PHP/`nil` in Ruby).

Examples:

Input `[1, 2, 3, 4]` --> Median `2.5`

Input `[3, 4, 1, 2, 5]` --> Median `3`

```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 = 'from random import choice\n\ndef median(lst):\n    return quickselect(lst, len(lst) // 2, len(lst) % 2 == 0)\n\ndef quickselect(lst, nth, even):\n    # O(n) but what a pain to get right.\n    pivot = choice(lst)\n    smaller = [n for n in lst if n < pivot]\n    n_equal = sum(n == pivot for n in lst)\n    bigger = [n for n in lst if n > pivot]\n    if even:\n        if len(smaller) > nth:\n            return quickselect(smaller, nth, True)\n        elif len(smaller) == nth:\n            return (pivot + quickselect(smaller, nth - 1, False)) / 2\n        elif len(smaller) + n_equal > nth:\n            return pivot\n        elif len(smaller) + n_equal == nth:\n            return (pivot + quickselect(bigger, nth - len(smaller) - n_equal, False)) / 2\n        else:\n            return quickselect(bigger, nth - len(smaller) - n_equal, True)\n    else:\n        if len(smaller) > nth:\n            return quickselect(smaller, nth, False)\n        elif len(smaller) + n_equal > nth:\n            return pivot\n        else:\n            return quickselect(bigger, nth - len(smaller) - n_equal, False)'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'median'
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: 7280