Everyone Knows AdFly and their Sister Sites.

If we see the Page Source of an ad.fly site we can see this perticular line:
Believe it or not This is actually the Encoded url which you would skip to.

The Algorithm is as Follows:
```
1) The ysmm value is broken like this

ysmm   = 0 1 2 3 4 5 6 7 8 9  = "0123456789"
code1  = 0   2   4   6   8    = "02468"
code2  =   9   7   5   3   1  = "97531"

2) code1+code2 is base64 Decoded

3) The result will be of this form :
https://adf.ly/go.php?u=

4)  has to be further decoded and the result is to be returned
```

Your Task:

```
Make a function to Encode text to ysmm value.

and a function to Decode ysmm value.
```

Note: Take random values for The first 2 int values.
I personally hate trivial error checking like giving integer input in place of string. 

```
Input For Decoder & Encoder: Strings
Return "Invalid" for invalid Strings
```

```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 = 'import base64\n\ndef adFly_decoder(sc):\n    #your code here\n    try:\n        code1 = sc[::2]\n        code2 = sc[1::2]\n        code2 = code2[::-1]    # reverse\n        decode_adfly = base64.b64decode(code1 + code2).decode("utf-8") \n        encoded_url = decode_adfly.split("https://adf.ly/go.php?u=")[-1]\n        return base64.b64decode(encoded_url).decode("utf-8") \n    except:\n        return "Invalid"\n    \ndef adFly_encoder(url):\n    #your code here\n    code = base64.b64encode(url.encode()).decode("utf-8")\n    ad_fly_code = "12https://adf.ly/go.php?u=" + code\n    final_code = base64.b64encode(ad_fly_code.encode()).decode("utf-8")\n    \n    l = len(final_code)\n    half = l // 2\n    first_half = final_code[0:half] \n    second_half = final_code[half if l % 2 == 0 else half+1:] \n    \n    second_half = second_half[::-1]\n    result_string = ""\n    \n    for i in range(half):\n        result_string += first_half[i]\n        if i < half - 1 or l % 2 == 0:\n            result_string += second_half[i]\n            \n    return result_string\n    \n'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'adFly_decoder'
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: 6485