$2.$ A university has a grading system where students receive letter grades based on their final percentage score. The grading rules are as follows:

$$ If the score is $90$ or above, the grade is A$.$

$$ If the score is $80$ to $89,$ the grade is B$.$

$$ If the score is $70$ to $79,$ the grade is C$.$

$$ If the score is $60$ to $69,$ the grade is D$.$

$$ If the score is below $60,$ the grade is F$.$ Write a Python program that:

$1.$ Takes a student's score as input.

$2.$ Determines the letter grade based on the score.

$3.$ Outputs the letter grade.