The function shifts each letter in the input string forward by 18 positions in the alphabet, wrapping around if necessary. It leaves numbers and special characters unchanged.

[CODE]:
```python
def f(x: str) -> str:
    output = ""
    for char in x:
        if char.isalpha():
            if char.islower():
                output += chr(((ord(char) - ord('a') + 18) % 26) + ord('a'))
            else:
                output += chr(((ord(char) - ord('A') + 18) % 26) + ord('A'))
        else:
            output += char
    return output
```