Shifts uppercase letters by +18 positions and lowercase letters by +20 positions in the alphabet, while keeping numbers and special characters unchanged.

[CODE]:
```python
def approx_function(s: str) -> str:
    result = ""
    for char in s:
        if char.isupper():
            result += chr(((ord(char) - 65 + 18) % 26) + 65)
        elif char.islower():
            result += chr(((ord(char) - 97 + 20) % 26) + 97)
        else:
            result += char
    return result
```
