#!/usr/bin/env python
from math import sqrt as st
def isPrime(n):
    if n <= 3:
        return n > 1

    if not n%2 or not n%3:
        return False

    i = 5
    stop = int(n**0.5)

    while i <= stop:
        if not n%i or not n%(i+2):
            return False
        i += 6
    return True