#include <iostream>
#include <fstream>
#define MAX 2000001
using namespace std;
ifstream fin("ciur.in");
ofstream fout("ciur.out");
char a[MAX];
int p[MAX-1];
int n,i,j,k;
void ciur()
{   fin>>n;
    for (i=2;i*i<=MAX;i++)
        if (a[i]==0)
        for (j=i*i;j<=MAX;j=j+i)
            a[j]=1;
    p[1]=2;p[2]=3;k=2;
    for (i=1;i<=n/6;i++)
    {if (a[i*6-1]==0)
        {k++;p[k]=i*6-1;}
    if (a[i*6+1]==0)
    {k++;p[k]=i*6+1;}

    }
    p[0]=k;


}




int main()
{   ciur();


fout<<p[0];

    return 0;
}