Exactly 3 Divisors

Exactly 3 Divisors 

Given a positive integer value N. The task is to find how many numbers less than or equal to N have numbers of divisors exactly equal to 3.

Example 1:

Input:
N = 6
Output: 1
Explanation: The only number with 
3 divisor is 4.

Example 2:

Input:
N = 10
Output: 2
Explanation: 4 and 9 have 3 divisors.

Your Task:
You don't need to read input or print anything. Your task is to complete the function exactly3Divisors() that takes N as parameter and returns count of numbers less than or equal to N with exactly 3 divisors.

Expected Time Complexity : O(N1/2 * N1/4)
Expected Auxilliary Space :  O(1)

Constraints :
1 <= N <= 109

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def isprime(n):

    if n==2:

        return True

    for i in range(2, int(n**0.5)+1):

        if n%i==0:

            return False

    return True

    

##Complete this function

def exactly3Divisors(N):

    ##Your code here

    n = int(N**0.5)

    c=0

    for i in range(2, n+1):

        if isprime(i):

            c+=1

    return c