线性筛

#include 
using namespace std;
const int N = 1000010;
int primes[N];
bool st[N];
int n, cnt;

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