#include <iostream>
#include <fstream>

using namespace std;

ifstream f("ciur.in");
ofstream g("ciur.out");

int main() {
	int n, cnt = 0;
	bool prim[500000];
	f >> n;
	for (int i = 2; i <= n; ++i) {
		prim[i] = 1;
	}
	for (int i = 2; i <= n; ++i) {
		if (prim[i]) {
			cnt++;
			for (int j = i*i; j <= n; j += i) {
				prim[j] = 0;
			}
		}
	}
	g << cnt;
}