An input string consisting of Roman alphabet characters, binary digits ("0" and "1"), operator symbols ("+", "-", and "*"), parentheses ("(" and ")"), 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.
def f(input_string: str):
    '''
    input_string: a string
    '''