An input string consisting of Roman alphabet characters, binary digits, operator symbols, parentheses, and equal signs. The input string has at most 31 characters.

### Example Input:

```
ACM
```

### Function Signature:
Write a function `f(input_string)` that takes in the input.
```python
def f(input_string: str):
    '''
    input_string: a string
    '''
```