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

using namespace std;
int N = 999999;
int prime[999999];
int status[999999/32];

bool Check(int N, int pos)
{
	return (bool)(N&(1<<pos));
}
int Set(int N, int pos)
{
	return N = N | (1<<pos);
}
void sieve()
{
	int i, j, sqrtN;
	sqrtN = sqrt(N);

	for(i=3; i<=sqrtN; i+=2)
	{
		if(Check(status[i/32], i%32) == 0)
		{
			for(j=i*i; j<=N; j+=(2*i))
				status[j/32] = Set(status[j/32], j%32);
		}
	}
	int k = 1;
	prime[0] = 2;
	for(int i=3; i<=N; i+=2)
	{
		if(Check(status[i/32], i%32) == 0)
			prime[k++] = i;
	}
}
int main()
{
	sieve();
	int k, num, count = 0;
	k=0;
    while(scanf("%d", &num)!=EOF)
    {
        k=0;
    	count = 0;

        for(int i=1; i<=num; i++)
        {
            if(i == prime[k])
                {
                    count++;
                    k++;
                }
        }
        printf("%d\n", count);
    }
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <fstream>
#include <algorithm>
#include <iomanip>
#include <map>
#include <queue>
#include <stack>
#include <set>
#include <ctime>
#include <iterator>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
using namespace std;

int main()
{
   vector<int> primes(999999+1,1);
   primes[0]=primes[1]=0;
   for(int c=2;c<primes.size();c++)
   {
       if(primes[c] == 0) continue;
       for(int c2=c+c;c2<primes.size();c2+=c) primes[c2]=0;
   }
   for(int c=1;c<primes.size();c++) primes[c]+=primes[c-1];

   int query;
   while(cin>>query) cout<<primes[query]<<endl;
}

C++: #include<iostream>
#define MAX 1000000
using namespace std;
int main(){
	int n, cnt;
	bool prime[MAX];	
	
	while(cin >> n){
		prime[n] ={false};
		prime[1] = true;
		
		// ??¨??????????????????????????????
		for(int j = 2; j*j <= n; j++){
			if(prime[j] == false){
				for(int k = j; k*j <= n; k++){
					prime[j*k] = true;
				}
			}
		}
		cnt = 0;
		//
		for(int i = 1; i <= n; i++){
			if(prime[i] == false) cnt++;
		}
		
		cout << cnt << endl;
	}
}

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

#define reep(i,n,m) for(int i=(n);i<(m);i++)
#define rep(i,n) reep(i,0,n)

using namespace std;

#define MAX 1000000

bool flg[MAX]={0};

void init(){
    flg[1] = true;
    reep(i,2,MAX){
        bool f=true;
        reep(k,2,sqrt(i)){
            if(i%k==0){
                f=false;
                break;
            }
        }
        if(f){
            for(int k=i+i;k<MAX;k+=i){
                flg[k]=true;
            }
        }
    }
}

int main(){
    init();
    int n;
    while(scanf("%d",&n)!=EOF){
        int ans=0;
        reep(i,1,n+1){
            if(!flg[i]){
                ans++;
            }
        }
        printf("%d\n",ans);
    }
}

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

int main(){
	int num;
	int count = 0;
	int i,j;

	cout << "数字を入力してください" << endl << "number :";
	cin >> num;

	for(i=2; i<num; i++){
		for(j=2; j<i; j++){
			if(i%j ==0){
				break;
			}
		}
		if(j == i){
			count++;
		}
	}
	cout << count << endl;
	return 0;
}

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

int main(){
  int a;
  int i, j;
  int past=1, a_sqrt;
  int count;
  bool chk[1000000];

  for(i=1; i<1000000; i++) chk[i]=true;
  while(cin >> a){
    count=0;
    if(past>=a){
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
    else {
      a_sqrt=(int)sqrt(a);
      for(i=2; i<=a_sqrt; i++){
        for(j=past+1; j<=a; j++){
          if(j%i==0) chk[j]=false;
        }
      }
      past=a;
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
  }

  return 0;
}

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

int Prime[1000000];

int main() {
	for(int i=0;i<1000000;++i)Prime[i]=1;
	
	Prime[0]=0;
	Prime[1]=0;
	
	for(int i=2;i<1000000;++i){
		if(Prime[i]){
			for(int j=i*2;j<1000000;j+=i){
				Prime[j]=0;
			}
		}
	}
	
	int n,cnt;
	while(cin>>n){
		cnt = 0;
		for(int i=0;i<=n;++i){
			cnt+=Prime[i];
		}
		cout<<cnt<<endl;
	}
	
	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <cstdio>
using namespace std;
int box[1000005];
int main()
{
	for(int i=0;i<=999999;i++)
		box[i]=1;
	box[1]=box[0]=0;
	int n;
    int rooted=sqrt(999999);
    for(int i=2;i<=rooted;i++)
        if(box[i])
            for(int j=2;j*i<=999999;j++)
                box[j*i]=0;
	while (cin >> n)
	{
		int s=0;
		for (int i=1;i<=n;i++)
			s+=box[i];
		cout << s << endl;
	}
}

C++: #include <stdio.h>

int main(void){
int n;
while(scanf("%d",&n)!=EOF){
int count=0;
for(int m=1;m<=n;m++){
bool check=true;
for(int i=2;i<m;i++){
if(i!=m && m%i==0){
chack=false;
}
}
if(check==true){
count++;
}
}
}
return 0;
}

C++: #include<iostream>
#include<string>
#include<new>
int main(){for(int m,n,o,c,d,e,i;!(std::cin>>n).eof();){if(n<3)e=n>1;else{bool*v=new bool[c=(n&1?n:--n)/2+1];memset(v,1,c);for(i=m=e=1;i<c;i++){for(m=i;m<c&&!v[m];)m++;if(m==c)break;e++;for(d=o=m*2+1;d<n+1;d+=o*2)v[d/2]=0;if(n<o*o)break;}for(i=1;i<c;)e+=v[i++];delete[]v;}printf("%d\n",e);}}

C++: #include<iostream>
using namespace std;
bool b[1000005];
int s[1000005];
int main(){
	int i,j,k;
	int n;
	b[i]=true;
	for(i=2;i<1000000;i++){
		if(b[i]==false){
			for(j=2*i;j<1000000;j+=i){
				b[j]=true;
			}
			s[i]=s[i-1]+1;
		}else{
			s[i]=s[i-1];
		}
	}
	while(cin>>n){
		cout<<s[n]<<endl;
	}
	return 0;
}

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

using namespace std;

int main() {
	bool a[1000000] = {1,1};
	int n, i, j, f = 0, count = 0;
	while (cin >> n) {
		count = 0;
		while (f <= ceil(sqrt(n))) {
			i = f + 1;
			while (a[i] == 1) {
				i++;
			}
			f = i;
			for (j = i + 1; j <= n; j++) {
				if (j % f == 0) a[j] = 1;
			}
		}
		for (i = 0; i <= n; i++) {
			if (a[i] == 0) { count++;}
		}
		cout << count << endl;
	}
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <limits.h>
#include <math.h>

#include <iostream>
#include <string>
#include <vector>
#include <iostream>
#include <sstream>
#include <fstream>
#include <string>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <algorithm>
#include <functional>
#include <utility>
#include <bitset>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <cstdio>

using namespace std;

#define FOR(i,s,e) for((i)=(s);(i)<(int)(e);(i)++)
long long ll;

const int N = 1000000;
bool prime[N];
int cnt[N];

bool isPrime(int n) {
	int i;
	for (i=2; i<n; i++)
		if (n%i == 0) return false;
	return n != 1;
}

void setPrime(int n) {
	int i, j;
	prime[0] = prime[1] = false;
	for (i=2; i<=n; i++) prime[i] = true;

	for (i=2; i<=n; i++) {
//		if (isPrime(i)) prime[i] = true;
		if (prime[i])
			for (j=i*2; j<=n; j+=i) prime[j] = false;
	}
}

int main() {
	int i, j;
	int n;

	setPrime(N);

	for (i=2; i<N; i++) {
		cnt[i] = cnt[i-1];
		if (prime[i]) cnt[i]++;
	}

	while (scanf("%d ", &n) != EOF) {
		printf("%d\n", cnt[n]);
	}

	return 0;
}