不用多说

#include <iostream>
using namespace std;

int n;
int prime[(int)(1e6 + 10)];

int main(){
    cin >> n;int cnt = 0;
    for (int i = 2; i <= n; i ++){
        if (!prime[i]){
            cnt ++;
            for (int j = i + i; j <= n; j += i) prime[j] = 1;
        }
    }
    cout << cnt;

    return 0;
}