The function swaps the second and third character in the input string. If the length of the input string is more than or equal to five characters, it adds 'S' as the fifth character.

[CODE]:
```python
def approx_function(input_str):
    if len(input_str) >= 3:
        input_str = input_str[:1] + input_str[2] + input_str[1] + input_str[3:]
        
    if len(input_str) >= 5:
        input_str = input_str[:4] + 'S' + input_str[4:]
        
    return input_str
```