#include <iostream>
using namespace std;
int main()
{
    int n;
    cin>>n;
    bool num[n+1]={0};
    int ans=0;
    for (long long i=2;i<=n;i++)
    {
        for (long long j=2;j*i<=n;j++)
        {
            num[i*j]=1;
        }
    }
    for (int i=2;i<=n;i++)
    {
        //cout<<i<<":"<<num[i]<<" ";
        if (num[i]==0)
        {
            ans++;
            //cout<<i<<" ";
        }
    }
    cout<<ans;
}

蒻蒟求助 另玄关是什么