埃氏筛

#include<iostream>
using namespace std;
const int N=1e8+1;
bool f[N];
void solution(int n){
    f[0]=f[1]=1;
    for (int i=2;i*i<=n;i++){
        if (f[i]==0){
            for (int j=i*i;j<=n;j+=i){
                f[j]=1;
            }
        }
    }
}
int main() {
    int n,sum=0;
    cin>>n;
    solution(n);
    for (int i=1;i<=n;i++){
        if (f[i]==0) sum++;
    }
    cout<<sum;
    return 0;
}