The function f(x) returns the middle part of the input string. If the input string has an odd length, it returns the middle character. If the input string has an even length, it returns the middle two characters.

[CODE]: 
```python
def approximate_function(s: str) -> str:
    length = len(s)
    if length % 2 == 0:
        return s[length // 2 - 1 : length // 2 + 1]
    else:
        return s[length // 2]
```