Cate numere prime sunt pana la n

#include <iostream>
#define N 100000000/2/8+1
using namespace std;
char p[N];
/// https://infogenius.ro/ciurul-lui-eratostene-cpp/
int Ciur(int n)
{
    int i,j,nr=1;
    for(i=1;((i*i)<<1)+(i<<1)<=n;i+=1)
    {
        if((p[i>>3]&(1<<(i&7)))==0)
        {
            for(j=((i*i)<<1)+(i<<1);(j<<1)+1<=n;j+=(i<<1)+1)
            {
                p[j>>3]|=(1<<(j&7));
            }
        }
    }
    for(i=1;2*i+1<=n;++i)
        if((p[i>>3]&(1<<(i&7)))==0)
            ++nr;
    return nr;
}

int main()
{
    int n;
    cin>>n;
    cout<<Ciur(n);
    return 0;
}