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