Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

C++: #include<iostream>
#include<vector>
#include<algorithm>
using namespace std;


vector<int> sieve_of_eratosthenes(int n) {
  vector<int> primes(n);
  for (int i = 2; i < n; ++i)
    primes[i] = i;
  for (int i = 2; i*i < n; ++i)
    if (primes[i])
      for (int j = i*i; j < n; j+=i)
        primes[j] = 0;
  return primes;
}

int main()
{
	int n;
	while(cin>>n){
		vector<int> V=sieve_of_eratosthenes(n+1);
		V.erase(remove(V.begin(),V.end(),0),V.end());
		cout<<V.size()<<endl;
	}
	return 0;
}

C++: 
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <vector>
#include <string>
#include <queue>
#include <algorithm>
#include <math.h>
#include <utility>
#include <set>
#include <map>
#define INF 100000
using namespace std;
typedef long long int ll;

int main(){
    vector<int> era(10000000,0);
    for (int i=1; i<era.size(); i++) {
        era[i]=1;
    }
    for (int i=2; i<era.size(); i++) {
        if (era[i]!=0) {
            int q = i + i;
            while (q<era.size()) {
                era[q]=0;
                q += i;
            }
        }
    }
    int n;
    while (cin >> n) {
        int ans=-1;
        for (int i=1; i<=n; i++) {
            if (era[i]) {
                ans++;
            }
        }
        cout << ans <<endl;
    }
}

C++: #include<stdio.h>
#include<math.h>
int isprime(int x){
	int i;
	if(x == 2){
 		return 1;
	}
  	if(x<2||x%2==0){
		return 0;
	}
	i = 3;
	while( i <= sqrt(x) ){
		if(x%i==0){
			return 0;
		}
		i = i + 2;
	}
	return 1;
}
int main(){
	int n;
	while(scanf("%d",&n)!=EOF){
		int c=0;
		for(int i=0;i<=n;i++){
			if(isprime(i)){
				c++;
			}
		}
		printf("%d\n",c);
	}
	return 0;
}

C++: 
#include<stdio.h>
#include<string.h>
#define range 999999
int total_added = 4;
struct primes{
	 int  key;
	 int  counts;
};
primes p[range];
bool is_prime(int key)
{
	int i;
	for (i = 2; i <= sqrt(key); i++)
	{
		if (key%i == 0)return false;
	}
	return true;
}
void init()
{
	p[0].key = 2;
	p[0].counts = 1;
	p[1].key = 3;
	p[1].counts = 2;
	p[2].key = 5;
	p[2].counts = 3;
	p[3].key = 7;
	p[3].counts = 4;
	int i;
	for (i = 8; i <= range; i++)
	{
		if (i % 2 != 0&&i%3!=0&&i%5!=0)
		{
			if (is_prime(i))
			{
				p[total_added].key = i;
				p[total_added].counts = p[total_added - 1].counts + 1;
				total_added++;
			}
		}
	}
}
int counting_result(int key)
{
	for (int i =total_added-1; i>=0; i--)
	{
		if (key >= p[i].key)
			return p[i].counts;
	}
	return 0;
}

int main()
{
	init();
	int i=0, n;
	while (i<30)
	{
		scanf("%d", &n);
		printf("%d\n",counting_result(n));
		i++;
	}

}

C++: #include<iostream>
#define N 1000001
using namespace std;
int main(){
  int n;
  int p[N]={0};
  for(int i=3;i<=N;i+=2) p[i] = 1;
  p[2] = 1;
  for(int i=3;i<=1000;i+=2){
    if(p[i]==1){
      for(int j=2*i;j<=N;j+=i) p[j] = 0;
    }
  }
  while(cin>>n){
    int count = 0;
    if(2<=n) count++;
    for(int i=3;i<=n;i+=2){
      if(p[i]) count++;
    }
    cout << count << endl;
  }
  return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;

int main(){
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n;
	while (cin >> n){
		int cnt=0;
		for (int i=2; i<=n; ++i){
			bool op=false;
			for (int j=2; j<=sqrt(i); ++j){
				if (i%j == 0){
					op = true;
					break;
				}
			}
			if (op == false) ++cnt;
		}
		cout << cnt << '\n';
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>

using namespace std;

int main()
{
    int n;

    while (cin >> n) {
        if (n == 2) {
            cout << 1 << endl;
            continue;
        }

        int ans = 1;
        for (int i = 3; i <= n; i += 2) {
            bool flag = true;
            
            if (!(i % 2))
                continue;
            for (int j = 3; j * j <= i; j += 2) {
                if (!(i % j)) {
                    flag = false;
                    break;
                }
            }
            if (flag)
                ans++;
        }
        cout << ans << endl;
    }
    
    return 0;
}

C++: #include<cstdio>
#include<vector>
#include<iostream>

using namespace std;

vector<bool> prime(1000010, 1);
int n;

void precalc()
{
    prime[0] = 0; prime[1] = 0; prime[2] = 1;
    for( int i = 2; i * i <= 999999; i++ ){
        if( prime[i] ){
            for( int j = 1; ( j + 1 ) * i <= 999999; j++ ){
                prime[i * ( j + 1 )] = 0;
            }
        }
    }

}

signed main(){
    precalc();
    while( scanf( "%d", &n ) != EOF )
    {
        int ret = 0;
        for( int i = 0; i <= n; i++ )
        {
            if( prime[i] )
            {
                ret++;
            }
        }
        printf( "%d\n", ret );
    }
    return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <vector>
#include <set>
#include <map>
#include <list>
#include <iterator>
#include <queue>
#include <stack>
#include <deque>
#include <sstream>
#include <algorithm>
#include <numeric>
#include <string>
#include <cmath>
#include <complex>
#include <functional>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cassert>
#include <cstdlib>
#include <cctype>

#define REP(i,m,n) for(int i=m;i<n;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define rp(i,c) rep(i,(int)c.size())
#define fr(i,c) for(__typeof((c).begin()) i=(c).begin();i!=(c).end();i++)
#define mp make_pair
#define pb push_back
#define all(c) (c).begin(),(c).end()
#define rall(u) (u).rbegin(),(u).rend()
#define dbg(x) cerr<<#x<<" = "<<(x)<<endl
#define v_delete(a,b) (a).erase(remove((a).begin(), (a).end(), b), (a).end())
#define v_unique(a) (a).erase(unique((a).begin(), (a).end()), (a).end())
#define VV(T) vector<vector< T > >
#define init(a,b) memset((a), (b), sizeof((a)))
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pi;
const int inf=1<<28;
const double INF=1e10,EPS=1e-9;


template<class T>int SIZE(T a){return a.size();}
template<class T>string IntToString(T num){string res;stringstream ss;ss<<num;return ss.str();}
template<class T>T StringToInt(string str){T res=0;for(int i=0;i<SIZE(str);i++)res=(res*10+str[i]-'0');return res;}
template<class T>T gcd(T a,T b){if(b==0)return a;return gcd(b,a%b);}
template<class T>T lcm(T a,T b){return a/gcd(a,b)*b;}

template <class T> void input(T& a, int n) { for (int i = 0; i < n; ++i) cin >> a[i]; }
template <class T> void input(T* a, int n) { for (int i = 0; i < n; ++i) cin >> a[i]; }
typedef pair<int,int> pii;

inline bool IN(int x, int y, int w, int h)
{
    return 0 <= x && x < w && 0 <= y && y < h;
}


int main()
{
	vi pn(1000000,1);
	int n;
	while(cin >> n){
	pn[0]=0;
	pn[1]=0;
	int ans=0;
	for(int i=2;i<=n;i++){
		if(pn[i]==1){
			for(int j=i+1;j<=n;j++){
				if(!(j%i)){
					pn[j]=0;
				}
			}
		}
	}
	rep(i,n+1){
		if(pn[i])ans++;
	}
	cout << ans << endl;
	}


}