Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

func next()string{
	return sc.Text()
}

func nextInt()int{
	a, _ := strconv.Atoi(next())
	return a
}

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

long length;
long prime[1000000];
bool isPrime(long n)
{
    bool isprimep = true;
    for(int i=0; i < length-1; i++)
        {
            if (!(n % prime[i]))
                {
                    isprimep = false;
                    break;
                }
        }
    return isprimep;
}

int main(void)
{
    long num;

    while(cin >> num)
    {
        length = 0;
        for (int n = 2; n <= num; n++)
        {
            if (isPrime(n))
            {
                prime[length] = n;
                length++;
            }
        }
        cout << length << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<set>
#include<map>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<cstdio>
#include<cstdlib>
using namespace std;
#define REP(i,a,n) for(int i = a ; i < n ; i++)
#define rep(i,n) REP(i,0,n)

typedef long long ll;

const int N = 1000000;  //MAXPRIME
const int M = 1000; //SQRT(N)
const int K = 600000;   //NUMBER OF PRIMES, CHOOSE 9/8 * N / LOG(N)

vector<int> iterativeSieve(){
  static int p[K],table[M];
  REP(i,2,M) p[i] = i;
  for(int i=2;i*i<M;i++)
    if(p[i])
      for(int j=i*i;j<M;j+=i)
        p[j] = 0;
  p[0] = p[1] = 0;

  int num = remove(p,p+M,0) - p;
  for(int m=M;m<N;m+=M){
    for(int x=m;x<m+M;x++)
      table[x-m] = x;
    for(int i=0,j;p[i]*p[i]<m+M;i++){
      if(p[i] >= m) j = p[i] * p[i];
      else if(m % p[i] == 0) j = m;
      else j = m - (m % p[i]) + p[i];
      for(;j<m+M;j+=p[i]) table[j-m] = 0;
    }
    num = remove_copy(table,table+M,p+num,0) - p;
  }
  return vector<int>(p,p+num);
}

int main(){
  vector<int> p = iterativeSieve();
  int n;
  while(cin>>n){
    cout<<upper_bound(p.begin(),p.end(),n) - p.begin()<<endl;
  }
}

C++: #pragma  warning (disable:4996)
#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<string>
#include<cmath>
#include<algorithm>
#define MAX 1000000
using namespace std;
int amount[MAX];
int vis[MAX];
int prime[MAX];
int main(){
	//memset(vis, 0, sizeof(vis));
	memset(prime, 1, sizeof(prime));
	prime[0] = prime[1] = 0;
	for (int i = 2; i*i<= MAX; i++){
		if (prime[i]){
			
			for (int j = i *i; j <= MAX; j += i){
				//vis[j] = 1;
				prime[j] = 0;
			}
		}
	}
	
	int n, m;
	while (cin >> n>>m){
		if (n==m) cout <<0 << endl;
		else {

		}
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <sstream>
#include <string>
#include <cstdlib>
using namespace std;

template <class T> int streamvar(std::istream& in, T& var)
{
    if (!in) {
        cerr << "invalid istream" << endl;
        return 1;
    }
    in >> var;
    if (in.fail()) {
        cerr << "no var" << endl;
        return 1;
    }
    return 0;
}
template <class T> int streamvec(std::istream& in, std::vector<T> &v)
{
    v.clear();
    string s;

    if (!in) {
        cerr << "invalid istream" << endl;
        return 1;
    }
    getline(in, s);
    if (in.fail()) {
        cerr << "no line" << endl;
        return 1;
    }
    istringstream is(s);
    while (1) {
        T tmp;
        is >> tmp;
        if (is.fail())
            break;
        v.push_back(tmp);
    }
    return 0;
}

#define NUM 1000000
int main(void)
{
    bool prime[NUM];
    int array[NUM] = {};
    for (int i = 0; i < NUM; i++) 
        prime[i] = 1;
    prime[0] = prime[1] = 0;

    for (int i = 0; i < NUM; i++) {
        if (prime[i]) {
            for (int j = i*2; j < NUM; j+=i) {
                prime[j] = 0;
            }
        }
    }
    array[0] = 0;
    for (int i = 1; i < NUM; i++) 
        array[i] = array[i-1] + prime[i];

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

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()
using namespace std;

int main(){
  int n;
  while(cin >> n){
    int ans=0;
    vector<int> vec;
    vec.push_back(2);
    for(int i = 3; i<=n; i+=2){
      bool isPrimeN = true;
      REP(j, vec.size()){
        if(i%vec[j]==0){
           false;
        }
      }
      for(int j=3; j<=floor(sqrt(i)); j++){
        if(i%j==0){
          isPrimeN = false;
          break;
        }
      }
      if(isPrimeN){
        vec.push_back(i);
      }
    }
    cout << vec.size() << endl;;
  }
  return 0;
}

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

int main()
{
   int n,count=0,m=0,i,j;
   
    while(scanf("%d",&n)!=EOF){
        for(i=2;i<=n;i++){
            for(j=1;j<i;j++){
                if(i%j==0)count++;
            }
            if(count==1)m++;
            count=0;
        }
        printf("%d\n",m);
        m=0;
    }

                return 0;
}

C++: #include <iostream>
using namespace std;
int a[1000000][2];
int main()
{
  int i,j,k,n;
  k = 0;
  for(i=2;i<1000000;i++){
    if(!a[i][0]){
      k++;
      for(j=2;i*j<1000000;j++){
	a[i*j][0] = 1;
      }
    }
    a[i][1] = k;
  }
  while(cin >> n){
    cout << a[n][1] << endl;
  }
  return 0;
}

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

using namespace std;

constexpr int MAX = 1000000;

int main()
{
    vector<bool> prime_list(MAX, true);
    prime_list.at(0) = false;
    prime_list.at(1) = false;
    for(int i = 2; i * i < MAX; ++i)
    {
        if(!prime_list.at(i))
            continue;
        for(int j = 2; i * j < MAX; ++j)
            prime_list.at(i * j) = false;
    }

    int num;
    int count;
    while(cin >> num)
    {
        count = 0;
        for(int i = 2; i <= num; ++i)
        {
            if(prime_list.at(i))
                count++;
        }
        cout << count << endl;
    }
}

C++: #include <stdio.h>

const int MAX = 999999;
int Prime[MAX];

void makePrimeList(int n) {
  Prime[0] = 1;
  int i, j;
  for (i = 2; i <= n / 2; i++) {
    for (j = 2; i * j <= n; j++) {
      Prime[i * j - 1] = 1;
    }
  }
}

int isPrime(int n) {
  if (Prime[n - 1] == 0) return 1;
  return 0;
}

int main() {
  
  int n, i, cnt, oldMax = 0;
  
  int init;
  for (init = 0; init <= MAX; init++) { Prime[init] = 0; }
  
  while(1) {
    if (scanf("%d", &n) == EOF) break;
    
    if (oldMax < n) {
      makePrimeList(n);
      oldMax = n;
    }
    cnt = 0;
    
    for (i = 1; i <= n; i++) {
      if (isPrime(i) == 1) cnt++;
    }
    
    printf("%d\n", cnt);
  }
  return 0;
}

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

int main(){
	bool prime[MAX_LIST];
	for(int i=0;i<MAX_LIST;i++){
		prime[i]=1;
	}
	prime[0]=prime[1]=0;
	for(int i=0;i*i<MAX_LIST;i++){
		if(prime[i]==1){
			for(int j=i*2;j<MAX_LIST;j=j+i){
				prime[j]=0;
			}
		}
	}
	int n;
	while(cin>>n){
		int count=0;
		for(int i=0;i<=n;i++){
			if(prime[i]==1){
				count++;
			}
		}
		cout<<count<<endl;
	}
	return 0;
}