import math


def isPrime(n):
    limit = n + 1
    primes = set()
    not_primes = set()
    primes.add(2)
    for num in range(3, limit):
        # if odd and not in primes yet
        if num % 2 and num not in not_primes:
            primes.add(num)
            for p in range(num * 2, limit, num):
                not_primes.add(p)
    return iter(primes)

def isRootPrime(n):
    root = math.sqrt(n)
    int_root = int(root)
    return isPrime(int_root)

def exactly3Divisors(N):
    # divideConquer(4, N, lambda num: 1 if isRootPrime(num) else 0)
    count = 0
    primes = isRootPrime(N)
    for prime in primes:
        if pow(prime, 2) <= N:
            count += 1
    return count