# 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