#include <bits/stdc++.h>

//https://www.jianshu.com/p/f16d318efe9b
using namespace std;

const int N = 1000000;
int check[N];//元素值为0代表是素数
int prime[N];

int main() {
    int cnt = 0;
    for (int i = 2; i < N; i++) {
        //如果是素数
        if (!check[i]) prime[cnt++] = i;
        for (int j = 0; j < cnt && i * prime[j] < N; j++) {
            check[i * prime[j]] = 1;//筛掉
            if (i % prime[j] == 0) break;
        }
    }
    printf("%.2f", (double) clock() / CLOCKS_PER_SEC);
    return 0;
}