noi 7827 质数的和与积

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <algorithm>

using namespace std;
int prime[10000],vis[10001];
void init(int n) {
    for(int i = 2;i <= n;i ++) {
        if(!vis[i]) prime[++ prime[0]] = i;
        for(int j = 1;j <= prime[0] && i * prime[j] <= n;j ++) {
            vis[i * prime[j]] = true;
            if(i % prime[j] == 0) break;
        }
    }
}
int main() {
    int s,m = 0;
    scanf("%d",&s);
    init(s);
    int l = 1,r = prime[0];
    while(l <= r) {
        while(l <= r && prime[r] + prime[l] > s) r --;
        if(prime[r] + prime[l] == s) {
            m = prime[r] * prime[l];
        }
        l ++;
    }
    printf("%d",m);
}