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++: // BCC
#include <cstdio>
#include <algorithm>
#include <iostream>
#include <utility>
#include <vector>
#include <queue>

// GCC
// #include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rrep(i, n) for (int i = (int)(n) - 1; i >= 0; i--)
#define each(i, c) for (__typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define chmin(a, b) a = min(a, b)
#define chmax(a, b) a = max(a, b)
#define pb push_back
#define mp make_pair

typedef long long ll;

const int INF = 1 << 28;
const ll INFLL = 1ll << 56;


/**
 * ??¨????????????????????????
 * ?´???°??¨??¨?´???°?????????????????????
 */

const int SIEVE_SIZE = 1000000;

bool is_prime[SIEVE_SIZE];
vector<int> primes;

void eratosthenes()
{
	primes.clear();
	for (int i = 2; i < SIEVE_SIZE; i++) is_prime[i] = true;
	
	for (int i = 2; i * i < SIEVE_SIZE; i++){
		if (!is_prime[i]) continue;
		for (int j = i * i; j < SIEVE_SIZE; j += i){
			is_prime[j] = false;
		}
	}
	
	for (int i = 2; i < SIEVE_SIZE; i++){
		if (is_prime[i]) primes.pb(i);
	}
}

int sum[1000000];

int main()
{
	eratosthenes();
	each(p, primes) sum[*p]++;
	rep(i, 999999) sum[i + 1] += sum[i];
	
	int n;
	while (~scanf("%d", &n)){
		printf("%d\n", sum[n]);
	}
}

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

int main(){
  bool is_prime[1000000];

  is_prime[0]=is_prime[1]=false;
  for(int i=2;i<1000000;i++){
    is_prime[i] = true;
  }

  for(int i=2;i*i<1000000;i++){
    if(is_prime[i]){
      for(int j=i*i;j<1000000;j=j+i){
        is_prime[j] = false;
      }
    }
  }

  int n_prime[1000000];

  int cnt = 0;
  for(int i=0;i<1000000;i++){
    if(is_prime[i]){
      cnt++;
    }
    n_prime[i] = cnt;
  }

  while(true){
    int n;
    cin >> n;

    if(cin.eof())
        break;

    cout << n_prime[n] << endl;
  }

  return 0;
}

C++: #include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#include <stdio.h>
using namespace std;
int isprime(int x){
    if(x==2||x==3) return true;
    if(x<5||x%2==0||x%3==0) return false;
    else{
        int i=5;
        while(i<=sqrt(x)){
            if(x%i==0) return false;
            i = i+2;
        }
        return true;
    }
}
int main() {
    int n;
    while(cin >> n){
        int ans=0;
    for (int j=n; j>1; j--) {
        if(isprime(j)!=0) ans++;
    }
    cout << ans << endl;
    }
}

C++: #include "bits/stdc++.h"

using namespace std;

typedef vector<int> vi;
typedef pair<int,int> pii;
typedef long long ll;

#define dump(x)  cerr << #x << " = " << (x) << endl
#define rep(i,n) for(int i=0;i<(n);i++)
#define all(a)  (a).begin(),(a).end()
#define pb push_back

bool era[10000000]={false};
 
 
void makeERA(){
    for(int i=2;i<1000001;i++){
        if(era[i]!=true)continue;
        for(int j=2;;j++){
            if(i*j<=1000000)
                era[i*j]=false;
            else break;
        }
    }
    era[2]=true;
}
int main(){
    rep(i,10000000)era[i]=true;
    era[0]=false;era[1]=false;
	makeERA();
	int n;
	while(cin>>n){
		if(n==0)break;

		int sum=0;

		rep(i,n+1){
			if(era[i])sum++;
		}
		cout<<sum<<endl;
	}
}

C++: #include <stdio.h>
#include <cmath>

using namespace std;

#define REP(a, i, n) for (int i = a; i < n; ++i)

bool isPrime(int n) {
    if (n == 1) return false;
    if (n == 2) return true;
    if (n % 2 == 0) return false;
    if (n == 3) return true;
    if (n % 3 == 0) return false;

    for (int i = 5; i <= sqrt(n); i += 2) {
        if (n % i == 0) {
            return false;
        }
    }
    return true;
}

//  n よりも小さい素数の数は何個存在するか
int main() {
    int n; // 10000
    scanf("%d", &n);

    int cnt = 0;
    REP(2, i, n) {
        if (isPrime(i)) cnt++;
    }

    printf("%d\n", cnt);

    return 0;
}

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

int a[999999]={};

int main(){
	int n,ans=0;
	vector<int> v;
	for(int i = 2; i*i < 999999; i++){
		for(int j = 0; j < 999999; j+=i){
			if(i!=j) a[j]=1;
		}
	}
	while(cin >> n){
		v.push_back(n);
	}
	for(int i = 0; i < v.size(); i++){
		for(int j = 2; j <= v[i]; j++){
			if(a[j]==0) ans++;
		}
		cout << ans << endl;
		ans=0;
	}
}

C++: #include <cmath>
#include <cstdio>
#include <map>

bool fermat_test(int n) {
    if (n < 2) {
        return false;
    } else if (n == 2) {
        return true;
    } else {
        return (static_cast<int>(std::pow(2, n - 1)) % n) == 1;
    }
}

bool prime_test(int n) {
    if (n < 2) {
        return false;
    } else if (n == 2) {
        return true;
    } else {
        for (int i = 3; i < static_cast<int>(std::sqrt(n)); i += 2) {
            if ((n %= i) == 0) {
                return false;
            }
        }
    }
    return true;
}

int main() {
    std::map<int, int> m;
    int n;
    while (~scanf("%d", &n)) {
        for (int i = 2; i <= n; ++i) {
            if (m.count(i) > 0)
                continue;

            m[i] = m[i - 1] + (fermat_test(i) && prime_test(i));
        }
        printf("%d\n", m[n]);
    }
}

C++: #include <iostream>
#include <cmath>
#include <algorithm>
#include <map>
#include <string>
#include <vector>
#define FOR(i, a, b)  for(int i=a; i<b; ++i)
#define rFOR(i, a, b) for(int i=a-1; i>=b; --i)
#define REP(i, n) for(int i=0; i<n; ++i)
#define rREP(i, n) for(int i=n-1; i>=0; --i)
#define all(a)  a.begin(), a.end()
#define MAXNUM 1000000
using namespace std;

int num[MAXNUM]={0};

int main(){
  int n;
  FOR(i, 2, (MAXNUM)/2){
    num[2*i]=1;
  }
  while(cin >> n){
    int ans=0;
    for(int i = 3; i<=n; i+=2){
      bool isPrimeN = true;
      if(num[i]!=0) continue;
      for(int j=3; j<=floor(sqrt(i)); j++){
        if(i%j==0){
          isPrimeN = false;
          break;
        }
      }
      if(isPrimeN){
        for(int j=2; j<=n; j++){
          if(j*i > n) break;
          num[j*i]=1;
        }
      }
    }
    FOR(i, 2, n+1){
      if(num[i]==0){
        ans++;
      }
    }
    cout << ans << endl;;
  }
  return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <set>
#include <map>
#include <cmath>
//ranker
using namespace std;

#define REPS(i, a, n) for (int i = (a); i < (n); ++i)
#define REP(i, n) REPS(i, 0, n)
#define RREP(i, n) REPS(i, 1, n + 1)
#define DEPS(i, a, n) for (int i = (a); i >= n; --i)
#define DEP(i, n) DEPS(i, n, 0)

using ll = long long;
using ull = unsigned long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using pii = pair<int, int>;
using pis = pair<int, string>;
using psi = pair<string, int>;
using D = double;


int main(){
    int n, ans, cnt, prime;

    while(cin >> n){
      prime = 1;
      if(n == 1) cout << 0 << endl;
      if(n == 2) cout << 1 << endl;
      if(n == 3) cout << 2 << endl;

      if(n >= 4){
      for(int i=3; i <= n; i++){
        cnt = 0;
        for(int j=3; j <= sqrt(i); j += 2){
          if(i%j == 0) cnt++;
        }
        if(cnt == 0  && i%2 != 0) prime++;
      }
      cout << prime << endl;
    }
    }
    return 0;
}