Question $3$: Conditional Challenges

Write a Python function named grade$\_$converter that takes a numerical grade $(0-100)$ as input and returns the corresponding letter grade based on the following scale:

$$ A: $90-100$

$$ B: $80-89$

$$ C: $70-79$

$$ D: $60-69$

$$ F: Below $60$

If the input is outside the range of $0$ to $100,$ the function should return "Invalid grade."