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++: #include<stdio.h>
#include<stdlib.h>
#include<iostream>
#include<algorithm>
#include<vector>
#include <numeric>
#include<queue>
#include <sstream>
#include<string.h>
#include<math.h>
using namespace std;
int prime[1000005];
bool is_prime[1000005];
int seve(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(void)
{
    int n;
    while(scanf("%d",&n)!=EOF)
    {
        printf("%d\n",seve(n));
    }
    return 0;
}

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

using namespace std;

bool p[3000000];
vector<int> prime;

void init(){
    memset(p,1,sizeof(p));
    p[0] = p[1] = false;
    for(int i = 2; i <= 1000000; i++){
        if(!p[i]) continue;
        for(long long j = 1LL*i*i; j <= 1000000; j += i)
            p[j] = false;
        prime.push_back(i);
    }
}

int CntPrime(int n){
    //vector<int>::iterator it = lower_bound(prime.begin(), prime.end(), n);
    //if( *it > n) it--;
    //return (it - prime.begin() + 1);
    int res = 0;
    for(int i = 0; i < prime.size(); i++){
        if(prime[i] <= n) res++;
        else break;
    }
    return res;
}

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

C++: #include <iostream>

using namespace std;

int main(){
	int n , ans = 1;

	cin >> n;

	if(n <= 1){
		ans--;
	}

	for(int i = 2; i <= n; i++){
		if(i % 2 != 0 && 9 > i){
			//cout << i << endl;
			ans++;

		}if(i >= 9){
			if(i % 2 != 0 && i % 3 != 0 && i % 5 != 0 && i % 7 != 0){
				//cout << i << endl;
				ans++;
			}
		}
	}

	cout << ans << endl;

	return 0;
}

C++: #include <iostream>
using namespace std;
#define x 1000000
int main(){
	bool q[x];
	for(int i=2;i<x;i++) q[i] = true;
	for(int i=2;i<x;i++) if(q[i]) for(int j=2;j<x/i;j++) q[i*j] = false;
	int n;
	while(cin >> n){
		int a = 0;
		for(int i=2;i<x;i++) if(q[i]) a++;
		cout << a << endl;
	}
}

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

int main(){
  int a,j,cou;
  while(cin>>a){
    cou=0;
    for(int i=2;i<=a;i++){
      if(i==2){
	cou++;
	continue;
      }
      else if(i%2==0)continue;
      for(j=3;j*j<=i;j+=2)if(i%j==0)break;
      if(j*j>i)cou++;
    }
    cout <<cou<<endl;
  }
  return 0;
}

C++: #include<iostream>
using namespace std;
int main() {
  int n;
  while(cin>>n) {
    if(n==1)
      cout<<0<<endl;
    else {
    int no=0;
    for(int i=2;i<=n;i++) {
      for(int j=2;j<i;j++) {
	if(i%j==0) {
	  no++;
	  j=n;
	}
      }
    }
    cout<<n-no-1<<endl; 
  }
  }
 return 0;
}

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

int main() {
	int N;
	while(cin >> N){
		int is_prime[N+1];
		is_prime[0]=0; is_prime[1]=0;
		for(int i=2;i<N+1; i++){
			is_prime[i]=1;
		}
		
		for(int s=2; s<N+1; s++){
		
		if(is_prime[s]==1){	
		for(int i=s*s; i<N+1; i+=s){
			is_prime[i]=0;
		}}
		
		}
		int ans=0;
		for(int i=2; i<N+1; i++){
			ans+=is_prime[i];
		}
		cout << ans <<endl;
		
	}
	return 0;
}

C++: /*#include"iostream"
#include"stdio.h"
#include"string.h"
using namespace std;
long kt( string a)
{
	long d=0, d1=0;
	long n=a.size();
	if(97<=a[0]&&a[0]<=122)
	{
			for(long i=1;i<n;i++)
			if(65<=a[i]&&a[i]<=90)
			d++;
	}
	
	for(long i=0;i<n;i++)
	{
		if(65<=a[i]&&a[i]<=90)
		d1++;
	}
	if(d1==n) return 2;
	if(d==n-1) return 1;
	return 0;
}
long main()
{
	
	string a;
	cin>>a;
	long n=a.size();
	if(kt(a)==2)
	{
		for(long i=0;i<n;i++)
		a[i]+=32;
		cout<<a;
		return 0;
	}
	if(kt(a)==1)
	{
		for(long i=1;i<n;i++)
		a[i]+=32;
		a[0]-=32;
		cout<<a;
		return 0;
	}
	else cout<<a;
}
*/

#include <iostream>
using namespace std;

#define long long long
const long N = 1000000;
bool np[N];
long p[N], pp=0;

void eratos() {
    np[0]=np[1]=true;
    for (long i=2; i*i<N; i++) if (!np[i])
        for (long j=i*i; j<N; j+=i) np[j]=true;
}
main()
{
	eratos();
	long n,d=0;
do
{
     d=0;
	cin>>n;
	
	for(long i=1;i<=n;i++)
	if(np[i]==false)
	d++;
	cout<<d<<endl; 

}while(1);
}

C++: #include <iostream>
using namespace std;
bool prime[1000000];
void sieve() {
    fill_n(prime, 1000000, true);
    for (int i=2; i<=1000 && prime[i]; ++i) {
        for (int j=i*i; j<1000000; j+=i) {
            prime[j] = false;
        }
    }
}
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    sieve();
    int n;
    while (cin >> n) {
        int cnt = 0;
        for (int i=2; i<=n; ++i) {
            if (prime[i]) {
                cnt ++;
            }
        }
        cout << cnt << "\n";
    }
    return 0;
}

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

using namespace std; 

int list[1000000]={2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73,
79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163,
 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251,
 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349,
 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443,
 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557,
 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647,
 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757,
 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863,
 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983,
 991, 997, 1009, 1013, 1019, 1021, 1031, 1033, 1039, 1049, 1051, 1061, 1063, 1069};

long int a,b,loop,hasil,input,mid;
long int lo;
long int hi;

 bool Prima(unsigned long n) 
{
    for (unsigned short i = 0; list[i]*list[i] <= n; i++) 
	{
        if (n % list[i] == 0) 
		{
            return false;
        }
    }
 
    return true;
}

int main()
{
	lo=1087;
	a=180;
	while(lo<=100003)
	{
		if(Prima(lo))
		{
			list[a]=lo;
			a++;
		}
		lo+=2;
	}
	//printf("%d",list[78498]);
	while(scanf("%ld",&input)!= EOF)
	{	
	lo=0;
	while(input>=list[lo])
	{
		lo++;
	}
	printf("%d\n",lo);
	}
	return 0; 
}

C++: #include <stdio.h>

int IsPrime(int n);

int main(void){
    int n, cnt, minID, min=1;
    int max[30], sol[30], listID=0;
	
    while (scanf("%d", &n) && n!=EOF){
        cnt=0;
        min = 1;
        minID = 0;
        for (int i=0; i<listID; i++){
            if (max[i] < n && min <max[i]){
                min = max[i];
				cnt = sol[i];
            }
        }
        for (int i=min+1; i<=n; i++){
            if (IsPrime(i)) cnt++;
        }
        max[listID] = n;
        sol[listID] = cnt;
        listID++;
        printf("%d\n", cnt);
    }
    return 0;
}

int IsPrime(int n){
    int i;
    for (i=2; i*i<=n; i++){
        if (n%i==0) return 0;
    }
    return 1;
}