#include <fstream>
#include <cmath>
using namespace std;

fstream cin("ciur.in");
ofstream cout("ciur.out");

bool is_prime(int x)
{
    bool prime = true;
    if (x % 2 == 0)
        prime = false;
    else
    {
        int sqx = sqrt(x);
        for (int i = 3; i <= sqx; i += 2)
        {
            if (x % i == 0)
            {
                prime = false;
                break;
            }
        }
    }
    return prime;
}
int y,x,k;
int main()
{
    cin >> y;
while(x!=y){x++;
if(is_prime(x) == true)k++;}
cout << k;
    return 0;
}