线性筛

#include <algorithm>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <vector>
//#include <tr1/unordered_map>
//#include <bits/stdc++.h>

#include <cmath>
//#include <unordered_map>
using namespace std;
#define sfd(i) scanf("%d", &i)
#define sfl(i) scanf("%I64d", &i)
#define sfs(i) scanf("%s", (i))
#define prd(i) printf("%d\n", i)
#define prl(i) printf("%I64d\n", i)
#define sff(i) scanf("%lf", &i)
#define ll long long
#define ull unsigned long long
#define uint unsigned int
#define mst(x, y) memset(x, y, sizeof(x))
#define INF 0x3f3f3f3f
#define inf 8e19
#define eps 1e-10
// const int maxn = 3e5;
#define PI acos(-1.0)
#define lowbit(x) ((x) & (-x))
#define fl() printf("flag\n")
#define MOD(x) ((x % mod) + mod) % mod
#define endl '\n'
#define pb push_back
#define lson (rt << 1)
#define rson (rt << 1 | 1)
#define FAST_IO \ ios::sync_with_stdio(false); \ cin.tie(0); \ cout.tie(0)

//-----------------------------------------------

const int N = 1e6 + 10;
bool st[N];

int primes[N], cnt;
//素数打表
void get_primes(int n)
{ 
   
    for(int i = 2; i <= n; i++)
    { 
   
        if(!st[i]) primes[cnt ++] = i;
        for(int j = 0; primes[j] <= n / i; j ++)
        { 
   
            st[primes[j] * i] = true;
            if(i % primes[j] == 0) break;
        }
    }
}

int main()
{ 
   
    int n;
    cin >> n;
    get_primes(n);
    cout << cnt << endl;
    return 0;
}