#include <iostream>

using namespace std;

void read (int&, char * const);
void mark (const int&, char * const);
void write (const int&, char * const);

int main ()
{
    char ciur[2000001];
    int n;
    read (n, ciur);
    mark (n, ciur);
    write (n, ciur);
    return 0;
}

void mark (const int &n, char * const ciur)
{
    int i, j;
    for (i = 2; i <= n; ++i)
    {
        if (ciur[i]) for (j = i * 2; j <= n; j += i)
        {
            ciur[j] = 0;
        }
    }
}

void read (int &n, char * const ciur)
{
    int i;
    freopen ("ciur.in", "r", stdin);
    cin >> n;
    for (i = 1; i <= n; i++)
    {
        ciur[i] = 1;
    }
}

void write (const int &n, char * const ciur)
{
    int i, sum = 0;
    for (i = 2; i <= n; ++i)
    {
        sum += ciur[i];
    }
    freopen ("ciur.out", "w", stdout);
    cout << sum << endl;
}