Caesars Cipher

One of the simplest and most widely known <dfn>ciphers</dfn> is a <dfn>Caesar cipher</dfn>, also known as a <dfn>shift cipher</dfn>. In a shift cipher the meanings of the letters are shifted by some set amount. A common modern use is the <a href="https://www.freecodecamp.org/news/how-to-code-the-caesar-cipher-an-introduction-to-basic-encryption-3bf77b4e19f7/" target="_blank" rel="noopener noreferrer nofollow">ROT13</a> cipher, where the values of the letters are shifted by 13 places. Thus `A ↔ N`, `B ↔ O` and so on. Write a function which takes a <a href="https://www.freecodecamp.org/news/how-to-code-the-caesar-cipher-an-introduction-to-basic-encryption-3bf77b4e19f7/" target="_blank" rel="noopener noreferrer nofollow">ROT13</a> encoded string as input and returns a decoded string. All letters will be uppercase. Do not transform any non-alphabetic character (i.e. spaces, punctuation), but do pass them on.