#include <iostream>
#include <vector>
using namespace std;

int main() 
{
	const int N = 2e7;
	
	vector <long long> is_prime(N, true);
	is_prime[0] = is_prime[1] = false;
	
	vector <long long> primes;
	
	for(long long i = 2; i < N; i++)
	{
	      if(is_prime[i]) primes.push_back(i);
	
	      for(int j = 0; j < primes.size() && i*primes[j] < N; j++)
	      {
	               is_prime[i*primes[j]] = false;
	
	               if(i%primes[j] == 0) break;
	      }
	}
	
	cout << primes.size();
		
	return 0;
}