#include <cmath>
#include <fstream>
using namespace std;
const int MAX_N = 21001000;
ifstream fin("ciur.in");
ofstream fout("ciur.out");
bool ciur[MAX_N];

int main() {
	int N;
	fin >> N;
	int numFound = 0;
	for (int i = 2; i <= N; ++i) {
		if (ciur[i] == 0) {
			for (int j = i * i; j <= N; j += i) {
				ciur[j] = 1;
			}
			++numFound;
		}
	}
	fout << numFound;
	return 0;
}