/*
 project euler problem 3: 找出 600851475143 的最大质因子。
 answer: 6857
*/

#include<bits/stdc++.h>
using namespace std;

typedef long long LL;

int main(void)
{
    LL num = 600851475143;
    LL ans = 2;
    for (LL i = 2; i * i < num; ++i)
    {
        while (num % i == 0) 
        {
            ans = i;
            num /= i;
        }
    }

    if (num > 1) ans = num;

    cout << ans << endl;
}