Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

C++: #include <iostream>
#include <cstdio>
#include <math.h>

using namespace std;

int prime[100000];
bool is_prime[1000000];

int sosuu(int n){
    int p=0;
    for (int i=0; i<=n; i++)
        is_prime[i]=true;
    is_prime[0]=is_prime[1]=false;
    for (int i=2; i<=n; i++) {
        if(is_prime[i]){
            prime[p++]=i;
            for (int j=i*2; j<=n; j+=i) {
                is_prime[j]=false;
            }
        }
    }
    return p;
}

int main(void){
    int n;
    while(cin>>n)
    cout<<sosuu(n)<<endl;
    return 0;
}

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

using namespace std;

bool prime_check(int n)
{
	if (n == 1)
	{
		return false;
	}
	else if (n == 2)
	{
		return true;
	}
	else
	{
		for (int i = 2; i <= sqrt(n); i++)
		{
			if (n % i == 0)
			{
				return false;
			}
		}
	}

	return true;
}

int main()
{
	// --- variable define --- //

	int n, c;


	// --- prime check ( 2 to 999999 ) --- //

	bool isprime[1000000];

	for (int i = 1; i < 1000000; i++)
	{
		if (prime_check(i) == true)
		{
			isprime[i] = true;
		}
		else
		{
			isprime[i] = false;
		}
	}


	// --- main --- //

	while (cin >> n)
	{
		c = 0;

		for (int i = 2; i <= n; i++)
		{
			if (isprime[i] == true)
			{
				c++;
			}
		}

		cout << c << endl;
	}
}

C++: #include <iostream>
#include <math.h>

int prime[1000000];


bool isPrime(int n) {
  int x = (int)sqrt(n);
  for (int i = 3; i <= x; i += 2) {
    if (n % i == 0) {
      return false;
    }
  }
  return true;
}


void get_prime(int max_num) {
  int primecount = 1;
  prime[2] = primecount;
  for (int i = 3; i <= max_num; i += 2) {
  ;
  }
}


int main () {
  int ns[31];
  int i = 0, max_num;
  while (std::cin >> ns[i]) {
    max_num = std::max(max_num, ns[i]);
    if (ns[i] % 2 == 0)
      ns[i] --;
    i ++;
  }
  get_prime(max_num);
  for (int j = 0; j < i; j ++) {
    std::cout << prime[ns[j]] << std::endl;
  }
  return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
	bool num[1000000] = {0};
	for(int i=2; i<1000; i++){
		if(!num[i]){
			for(int j=i*2; j<1000000; j+=i){num[j] = true;}
		}
	}
	int n;
	while(cin >>n){
		int f = 0;
		for(int i=2; i<=n; i++){
			if(!num[i]){f++;}
		}
		cout <<f<<endl;
	}
}

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

int main()
{
	int n, i,j,k,cnt;
	while (scanf("%d", &n) != EOF)
	{
		cnt = 0;
		if (n <= 1)
		{
			printf("0\n");
			continue;
		}
		if (n == 2)
		{
			printf("1\n");
			continue;
		}
		if (n <= 4)
		{
			printf("2\n");
			continue;
		}
		for (j = 5; j <= n; j+=2)
		{
			k = 0;
			for (i = 3; i <= sqrt(j*1.0); i += 2)
			{
				if (j%i == 0)
				{
					k = -1;
					break;
				}
			}
			if (k == -1)continue;
			cnt++;
		}
		printf("%d\n", cnt + 2);
	}
}

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

using namespace std;

    int g(int n){
        int p = 0;
        bool is_prime[n + 1];
        is_prime[0] = is_prime[1] = false;

        for(int i = 2 ; i <= n ; i++)
            is_prime[i] = true;

        for(int i = 2 ; i <= n ; i++){
            if(is_prime[i]){
                p++;
                for(int j = i * 2 ; j <= n ; j += i) // j = i * 2 like 2 started form 4 then add 2 each time j += i not j * 2
                    is_prime[j] = false;
            }
        }
    return p;
}

int main(){
    int n;

    while(cin >> n){
        cout << g(n) << endl;
    }

}

C++: #include<iostream>
using namespace std;
int main(){
	int n, cnt;
	bool prime[1000000] = false;
	prime[1] = true; //1???????????\
	
	while(cin >> n){
		// ??¨??????????????????????????????
		for(int j = 2; j*j <= n; j++){
			if(prime[j] == false){
				for(int k = j; k*j <= n; k++){
					prime[j*k] =true;
				}
			}
		}
		
		for(int i = 1; i <= n; i++){
			if(prime[i] == false) cnt++;
		}
		
		cout << cnt << endl;
	}
}

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

int main(){
	int n;
	while(cin >> n && n<1000000) {
		int s = 0;
			for (int j = 2;j <= n;j++){
				int yaku = 0;
				for (int m = 1;m <= j;m++){
					if (j % m == 0) yaku++;
				}
				if (yaku == 2) s++;
			}
			cout << s << endl;
	}
    return 0;
}

C++: 
#include <iostream>
#include <cmath>
#include <vector>
#include <string>
#include <map>
#include <algorithm>
#include <tuple>
#include <set>

using namespace std;
using ll = long long;
using ull = unsigned long long;
#define out(S) cout<<(S)<<endl;
#define REP(i,b) for(size_t i=0;i<(b);i++)
#define rREP(i,b) for(int i=(b)-1;i>=0;i--)
#define FOR(i,a,b) for(size_t i=(a);i<(b);i++)
#define rFOR(i,a,b) for(int i=(b)-1;i>=0;i--)
#define Foreach(item,collection) for(auto item:collection)
#define mod(i) ((i) % (ll)(1e9 + 7))
ll modpow(ll i, ll j) { ll tmp = 1; while (j) { if (j % 2)tmp = mod(tmp*i); i = mod(i*i); j /= 2; }return tmp; }
#define divmod(a,b) (mod(a * modpow((ll)b,(ll)(1e9 + 5))))
#define Yes out("Yes")
#define No out("No")
#define NO out("NO")
#define YES out("YES")
#define INF 1e10;
#define ShowAll(collection) for(auto i:collection){out(i);}
#define IfOut(condition,text) if((condition)){out(text);return 0;}
#define IfeOut(condition,itext,etext) if(condition){out(itext);return 0;}else{out(etext);return 0;}
#define Select(collection,condition,result) Foreach(i,collection){result+=condition;}
#define GetAuto(_N,vect) {cin>>N;vect.resize(N);REP(i,_N){cin>>vect[i];}}
void removeAt(string& s, int index) { s.erase(index, 1); }
template<typename T> void removeAt(vector<T>& v, int index) { v.erase(v.begin() + index); }
ll manhattanDistance(ll x1, ll y1, ll x2, ll y2) { return (abs(x2 - x1) + abs(y2 - y1)); }
#define beginend(vec) (vec).begin(),(vec).end()
vector<ll> sieveOfEratosthenes(int max) {ll prime;double sqrtmax = sqrt(max);vector<ll> primeVec, searchVec;FOR(i, 2, max + 1) searchVec.push_back(i);do{prime = searchVec[0];primeVec.push_back(prime);auto itr = remove_if(beginend(searchVec), [=](ll x)->bool {return x%prime == 0; });searchVec.erase(itr, searchVec.end());} while (prime < sqrtmax);primeVec.reserve(primeVec.size() + searchVec.size());primeVec.insert(primeVec.end(), beginend(searchVec));return primeVec;}


int main() {
	ll n;
	vector<ll> ans;
	while (cin >> n)
	{
		if (n == 1) {
			ans.push_back(0);
			continue;
		}
		int prime;
		double sqrtmax = sqrt(n);
		vector<ll> primeVec, searchVec;
		FOR(i, 2, n + 1) searchVec.push_back(i);
		do
		{
			prime = searchVec[0];
			primeVec.push_back(prime);
			auto itr = remove_if(beginend(searchVec), [=](ll x)->bool {return x%prime == 0; });
			searchVec.erase(itr, searchVec.end());
		} while (prime < sqrtmax);
		ans.push_back(primeVec.size() + searchVec.size());
	}
	Foreach(i, ans) out(i);
	return 0;
}

C++: #include <iostream>

int main() {
	int n;
	while (std::cin >> n) {
		int c = 0;
		for (int i = 2; i <= n; i++) {
			bool fg = true;
			for (int j = 2; j < i; j++)
				if (!(i % j))
					fg = false;
			if (fg)
				c++;
		}
		std::cout << c << std::endl;
	}
}

C++: #include <iostream>
#define MAX_N 1000000
using namespace std;

int prime[MAX_N];
bool is_prime[MAX_N+1];

int sieve(int n) {
  int p = 0;
  for(int i=0;i<=n;i++) {
    is_prime[i] = true;
  }
  is_prime[0] = is_prime[1] = false;
  for(int i=2;i <= n;i++) {
    if(is_prime[i]) {
      prime[p++] = i;
      for(int j=2 * i;j<= n;j += i) is_prime[j] = false;
    }
  }
  return p;
}

int main() {
  int n;
  cin >> n;
  cout << sieve(n) << endl;
}