筛完质数就差不多做完了

#include <iostream>
#include <stack>
#include <cmath>
#include <vector>
#include <string.h>
#include <queue>
#include <stdio.h>
#include <iomanip>
#include <cstdio>
#include <algorithm>
#define int long long
using namespace std;
const int N = 1e5 + 10;
const int INF = 0x3f3f3f3f;
int s;
bool prime[N];
void find_prime(int x)
{
    prime[1] = 1;
	for(int i = 2; i * i <= x; i++)
	{
		if(prime[i] == 0)
		{
			for(int j = i * i; j <= x; j += i)
			{
				prime[j] = 1;
			}
		}
	}
}
signed main()
{
    int s;
    cin >> s;
    find_prime(s);
    if(s % 2 == 1)
    {
        cout << 2 * (s - 2) << endl;
        return 0;
    }
    int maxx = -INF;
    for(int i = 1; i <= s; i++)
    {
        if(prime[i] == 0)
        {
            if(prime[s - i] == 0)
            {
                maxx = max(maxx, i * (s - i));
            }
        }
    }
    cout << maxx << endl;
	return 0;
}