def dencrypt(s: str, n: int = 13) -> str:
 """
 https://en.wikipedia.org/wiki/ROT13

 >>> msg = "My secret bank account number is 173-52946 so don't tell anyone!!"
 >>> s = dencrypt(msg)
 >>> s
 "Zl frperg onax nppbhag ahzore vf 173-52946 fb qba'g gryy nalbar!!"
 >>> dencrypt(s) == msg
 True
 """
 out = ""
 for c in s:
 if "A" <= c <= "Z":
 out += chr(ord("A") + (ord(c) - ord("A") + n) % 26)
 elif "a" <= c <= "z":
 out += chr(ord("a") + (ord(c) - ord("a") + n) % 26)
 else:
 out += c
 return out


def main() -> None:
 s0 = input("Enter message: ")

 s1 = dencrypt(s0, 13)
 print("Encryption:", s1)

 s2 = dencrypt(s1, 13)
 print("Decryption: ", s2)


if __name__ == "__main__":
 import doctest

 doctest.testmod()
 main()