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++: /*
C++ ???????????????????????????????????¬??????
??????AOJ-ITP1??????????????????
*/
#define _USE_MATH_DEFINES

#include <iostream> // ?¨??????\??????
#include <fstream>  // ??????????????\??????
#include <string>   // ???????????¢???
#include <vector>   // ???????????????
#include <cmath>    // ??°??????
#include <iomanip>  // stream???????????¢???
#include <algorithm>// max??¢??°??????

using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;        // ????´???°????????????????????????
using std::fixed;         // ???????????????????¨???????????????????????????????
using std::setprecision;  // ?????°????°???°?????\???????????°????????????????????¨????????\???

int main() {
	int a;
	int count;
	int ccount;
	vector<int> c = { 0 };
	while (cin >> a)
	{
		a = a + 1;
		count = 0;
		/*
		// ?´???°?????????
		// ??????????????°
		for (size_t i = 2; i < a; i++)
		{
			ccount = 0;
			// ????????°
			for (size_t j = 1; j <= i; j++)
			{
				//cout << "i= " << i << "j= " << j << endl;
				if (i % j == 0) {
					ccount++;
				}
			}
			if (ccount > 2) {
				count++;
			}
		}*/
		// ?????§????????§????????????0??§???????????????
		c.resize(a);

		
		// 2*i????????§????????§?????????????????????1?????????
		for (size_t i = 2; i < a; i++)
		{
			for (size_t j = 2; i*j < a; j++)
			{
				//cout << i*j << endl;
				c[i*j] = 1;
			}
		}
		// ??????????????????0??????????????°????????????
		for (size_t i = 0; i < a; i++)
		{
			if (c[i] == 0) {
				count++;
			}
		}
		cout << count-2 << endl;

		// ?¢??????¨
		/*
		for (size_t i = 0; i < a; i++)
		{
			cout << i << " = " << c[i] << endl;
		}
		*/
		
	}
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;



int main(){

  int n;

  bool p[999999];
  int head;
  int sum;

  while(scanf("%d",&n)!=EOF){

    head = 2;
    sum = 0;

    for(int i=0; i<n; i++){ p[i]=false; }

    while(head<=n){
      
      while(head<=n && p[head-1]==true){ head++; }

      if(!(head>n))sum++;

      for(int i=2; i*head<=n; i++){p[i*head-1]=true;}

      head++;
    }

    cout << sum << endl;
      

  }


  return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <string>
#include <array>
#include <queue>
#include <deque>
#include <set>
#include <list>
#include <map>
#include <stack>
#include <utility>
#include <algorithm>
#include <numeric>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cctype>
#include <cstring>
#include <climits>
#include <bitset>
#include <random>
#include <functional>
#include <sstream>
#include <iomanip>

using namespace std;

#define _CRT_SECURE_NO_WARNINGS
#define rep(i, n) for(int i=0; i<(n); i++)
#define FOR(i, m, n) for(int i=(m);i<(n);i++)
#define SZ(x) ((int)(x).size())
#define all(x) (x).begin(),(x).end()
#define SORT(x) sort((x).begin(),(x).end())
#define REVE(x) reverse((x).begin(),(x).end())
#define mp make_pair
#define pb push_back

typedef vector<int> VI;
typedef vector<string> VS;
typedef vector<vector<int>> VVI;
typedef pair<int, int> PII;
typedef long long LL;

long long modpow(long long x, long long n, long long m) {
	long long ans = 1;
	while (0 < n) {
		if (n % 2 == 0)x = x * x%m, n >>= 1;
		else ans = ans * x%m, n--;
	}
	return ans;
}

vector<long long> v = { 2,7,61 };

//ミラー-ラビン素数判定法
bool Miller_Rabin(long long n) {
	if (n < 2)return false;

	//(n-1)を(2^s*d)の形にする
	long long d = n - 1, s = 0;
	while (d % 2 == 0)
		d /= 2, s++;

	//aを2,7,61として調べる
	for (auto a : v) {
		if (a == n)return true;

		bool flag = false;
		long long r = modpow(a, d, n);

		if (r == 1 || r == n - 1)flag = true;
		for (int j = 0; j < s; j++) {
			r = modpow(r, 2, n);
			if (r == n - 1)flag = true;
		}
		if (!flag)return false;
	}
	return true;
}

int main() {
	VI p;
	for (int i = 2; i < 1000000; i++) {
		if (Miller_Rabin(i))p.pb(i);
	}
	int n;
	while (cin >> n) {
		decltype(p)::iterator it = lower_bound(all(p), n);
		size_t pos = distance(p.begin(), it);
		if (p[pos] == n)pos++;
		cout << pos << endl;
	}
}

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

const int MAXN = 1000000;

int noprime[MAXN];
bool isprime[MAXN];

void sieve()
{
	int p = 0;
	for (int i = 0; i < MAXN; i++)
		isprime[i] = true;
	isprime[0] = isprime[1] = false;
	for (int i = 2; i < MAXN; i++)
	{
		if (isprime[i])
		{
			p++;
			for (int j = 2 * i; j <= MAXN; j += i)
				isprime[j] = false;
		}
		noprime[i] = p;
	}
}

int main()
{
	sieve();
	int n;
	while (cin >> n)
		cout << noprime[n] << endl;
	return 0;
}

C++: // aoj.cpp : コンソール アプリケーションのエントリ ポイントを定義します。
//

#include "stdafx.h"


#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <iostream>

#define N 1000000L
bool num[N];

void furui() {
	long int i;
	int divide;
	int rt = 1000;
	for (i = 0L; i < N; i++) {
		num[i] = true;
	}
	num[0] = false;
	num[1] = false;
	for (divide = 2; divide <= rt; divide++) {
		if (num[divide] == false) continue;
		for (i = (long int)divide; i < N; i += divide) {
			num[i] = false;
		}
		num[divide] = true;
	}
}

int main(void) {
	int c, i, j;
	long int k;
	char buf[7];
	long int n;
	long int count;

	furui();

	for (j = 0; j < 30; j++) {
		for (i = 0; ; i++) {
			c = getchar();
			if (c == EOF || c == '\n') {
				buf[i] = '\0';
				break;
			}
			buf[i] = c;
		}
		n = atol(buf);
		count = 0L;
		for (k = 1L; k <= n; k++) {
			if (num[k] == true) count++;
		}
		std::cout << count << std::endl;
		if (c == EOF) break;
	}
	return 0;
}

C++: #include <vector>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <algorithm>
#include <utility>
#include <functional>
#include <sstream>
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <climits>
using namespace std;
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v;}
template<class T> inline string toStr(T x) { ostringstream sout; sout << x; return sout.str();}
typedef vector<int> vi;
typedef vector<vi>  vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define FOR(i,a,b) for(int i=(a);i<=(b);++i)
#define REP(i,n) FOR(i,0,(n)-1)
const double EPS = 1e-12;
const double PI = acos(-1.0);
const int INF = INT_MAX/10;

int main() {
	vi prime(1000001, 1);
	prime[0] = prime[1] = 0;
	FOR(i, 2, 1000) {
		if(prime[i]) {
			for(int j = i*2; j <= 1000000; j += i) {
				prime[j] = 0;
			}
		}
	}

	int n;
	while(cin >> n) {
		int ans = 0;
		REP(i, n+1) {
			ans += prime[i];
		}
		cout << ans << endl;
	}

	return 0;
}