===== ISTEM =====
Solve the following programming task.

TASK: Roman Numeral to Integer

Write a function 'roman_to_int(s)' that converts a valid Roman numeral string (I, V, X, L, C, D, M) to an integer. It must handle the rules IV=4, IX=9, XL=40, XC=90, CD=400, CM=900.

STARTER CODE (python):
def roman_to_int(s: str) -> int:
    # Write your code here
    pass


RULES:
- Keep the function name and signature EXACTLY as given.
- Return working code only. No explanations.
- Put the code in a single ``` block.
- Try to solve it first. If you genuinely cannot, write only this single
  line instead of producing faulty code: CANNOT_SOLVE

===== HAM YANIT =====
```python
def roman_to_int(s: str) -> int:
    values = {'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}
    total = 0
    n = len(s)
    for i in range(n):
        v = values[s[i]]
        if i + 1 < n and values[s[i + 1]] > v:
            total -= v
        else:
            total += v
    return total
```