#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;
ifstream fin("ciur.in");
ofstream fout("ciur.out");
int a[2000000],n;

void ciur()
{
    int i,j;
    for (i=2;i<=sqrt(n);i++)
        if (a[i]==0)
            for (j=i*i;j<=n;j+=i)
                a[j]=1;
}

int contor()
{
    int ct=0,i;
    for (i=1;i<=n/2;i++)
        if (a[i]==0)
            ct++;
    return ct;
}

int main()
{
    fin>>n;
    ciur();
    fout<<contor();
    return 0;
}