is there someone who can explain me this question?

i googled about cardinality and i got to know that it means the no of elements in set.

question:

Take an integer n < 1024 as input from user. Write a function to compute the value of the following function. f(n) = |{a|gcd(a,n) = 1,1 ≤ a ≤ n}|

In words, f(n) is the cardinality of the set of numbers a for which gcd(a,n) = 1, where 1 ≤ a ≤ n. Your function should only take n as input and return f(n). Assume both input and return values for the function are integers.

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