For encrypting strings this region of chars is given (in this order!):

* all letters (ascending, first all UpperCase, then all LowerCase)
* all digits (ascending)
* the following chars: `.,:;-?! '()$%&"` 

These are 77 chars! (This region is zero-based.)

Write two methods: 
```python
def encrypt(text)
def decrypt(encrypted_text)
```

Prechecks:
1. If the input-string has chars, that are not in the region, throw an Exception(C#, Python) or Error(JavaScript).
2. If the input-string is null or empty return exactly this value!

For building the encrypted string:
1. For every second char do a switch of the case.
2. For every char take the index from the region. Take the difference from the region-index of the char before (from the input text! Not from the fresh encrypted char before!). (Char2 = Char1-Char2)
Replace the original char by the char of the difference-value from the region. In this step the first letter of the text is unchanged.
3. Replace the first char by the mirror in the given region. (`'A' -> '"'`, `'B' -> '&'`, ...)

Simple example:

* Input:  `"Business"`
* Step 1: `"BUsInEsS"`
* Step 2: `"B61kujla"`
  * `B -> U`
    * `B (1) - U (20) = -19`
    * `-19 + 77 = 58`
    * `Region[58] = "6"`
  * `U -> s`
    * `U (20) - s (44) = -24`
    * `-24 + 77 = 53`
    * `Region[53] = "1"`
* Step 3: `"&61kujla"`

This kata is part of the Simple Encryption Series:
Simple Encryption #1 - Alternating Split
Simple Encryption #2 - Index-Difference
Simple Encryption #3 - Turn The Bits Around
Simple Encryption #4 - Qwerty

Have fun coding it and please don't forget to vote and rank this kata! :-)

```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 encrypt(text):\n\n    import string\n\n    #prechecks\n\n    char_filter = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.,:;-?! \'()$%&" + \'"\'\n    char_filter = [char for char in char_filter]\n    \n    if text == "" or text is None:\n        return text\n    else:\n        for char in text:\n            if char not in char_filter:\n                raise Exception("text contains invalid character")\n\n        #turn text to list\n        txt_list = [char for char in text]\n\n        #step 1\n        for charIndex in range(1, len(txt_list), 2):\n\n            if txt_list[charIndex] in string.ascii_uppercase:\n                txt_list[charIndex] = txt_list[charIndex].lower()\n\n            elif txt_list[charIndex] in string.ascii_lowercase:\n                txt_list[charIndex] = txt_list[charIndex].upper()\n\n        #step 2\n        for i in range(len(txt_list)-1):\n            #x = len-i makes it go through list in reverse order\n            x = len(txt_list) - 1 - i\n            char2 = char_filter.index(txt_list[x])\n            char1 = char_filter.index(txt_list[x - 1])\n            txt_list[x] = char_filter[char1-char2]\n\n        #step 3\n        char = char_filter.index(txt_list[0])\n        txt_list[0] = char_filter[char * - 1 - 1]\n\n        #convert list to text\n        text = ""\n        for char in txt_list:\n            text += char\n\n        return text\n    \n\ndef decrypt(text):\n\n    import string\n\n    #prechecks\n\n    char_filter = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.,:;-?! \'()$%&" + \'"\'\n    char_filter = [char for char in char_filter]\n    if text == "" or text is None:\n        return text\n    else:\n        for char in text:\n            if char not in char_filter:\n                raise Exception("text contains invalid character")\n    \n        #turn text to list\n        txt_list = [char for char in text]\n\n        #step 3 reverse\n        txt_list[0] = char_filter[77 - char_filter.index(txt_list[0]) - 1]\n\n        #step 2 reverse\n        for i in range(1, len(txt_list)):\n            txt_list[i] = char_filter[char_filter.index(txt_list[i - 1]) - char_filter.index(txt_list[i])]\n\n        #step 1 reverse\n\n        for charIndex in range(1, len(txt_list), 2):\n\n            if txt_list[charIndex] in string.ascii_uppercase:\n                txt_list[charIndex] = txt_list[charIndex].lower()\n\n            elif txt_list[charIndex] in string.ascii_lowercase:\n                txt_list[charIndex] = txt_list[charIndex].upper()\n\n        #convert list to text\n        text = ""\n        for char in txt_list:\n            text += char\n\n        return text'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'decrypt'
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: 9229