[split] problem with function return value

[ops]

HELP ME TO SOLVE THIS problem USING FUNCTION
A teacher in a school wants to find and display the grade of a student based on his/her percentage score. The criterion for grades is as given below:

Score (both inclusive) Grade
Between 80 and 100 A
Between 73 and 79 B
Between 65 and 72 C
Between 0 and 64 D
Any other value Z
Assume that a percentage score is a whole number.
AND I HAVE WRITTEN THIS, BUT ITS NOT RETURNING ANY VALUE
MY CODE IS LIKE THIS

num=0
num=input('enter your score=')
#Start writing your code here
def score(num):
    if num>=80 or num<=100:
        n='A'
    elif num>=73 or num<=79:
        n='B'
    elif num>=65 or num<=72:
        n='c'
    elif num>=0 or num<=64:
        n='d'
    else:
        n='e'
    return n
    num=input('enter your score=')
    print(score)