# Problem_3.py

"""

The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?

600851475143的最大质因数

"""

from math import sqrt

num = 600851475143

size = int(sqrt(num)) + 1

is_prime = [True for i in range(size)]

for i in range(2, size):

    if is_prime[i]:

        j = 2

        while i * j < size:

            is_prime[i * j] = False

            j += 1

for i in range(size - 1, 1, -1):

    if is_prime[i] and num % i == 0:

        print(i)

        break