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>
using namespace std;

int main(){
	bool *map;
	int n, cnt=0;

	cin >> n;
	map = new bool[n+1];
	for( int i=2; i<=n; i++ ){
		map[i] = true;
	}

	for( int i=2; i<=n/2; i++ ){
		for( int j=2*i; j<=n; j+=i ){
			if( map[j] ){
				map[j] = false;
			}
		}
	}

	for( int i=2; i<=n; i++ ){
		if( map[i] ){
			cnt++;
		}
	}
	cout << cnt << endl;

	delete[] map;
	map = 0;
}

C++: #include <iostream>
using namespace std;
int prime[10005]= {0};
int nprime=0;
void get_prime()
{
    for(int i=2; i<10005; i++)
    {
        if(!prime[i])
        {
            for(int j=i*i; j<10005; j+=i)

            {
                prime[j]=1;
            }
        }
    }
}
void print()
{
    for(int i=1; i<10005; i++)
    {
        if(prime[i]==0)
        {
            cout <<i << " ";

        }

    }
}
int main ()
{
    int n ;
    get_prime();
    while(cin >> n )
    {
        int sum =0;
        for(int i=2; i<=n; i++)
        {
            if(prime[i]==0)
            {
                sum ++ ;
            }
        }
         cout <<sum << endl ;

    }



//        print();
}

C++: #include<iostream>
#include<stdio.h>
#include<windows.h>
#include<conio.h>
#include<String>
#include<time.h>
#include<math.h>

using namespace std;
int c = 0;
int i = 1;
unsigned long long a = 0;
unsigned long long b = 0;
unsigned long long sosuu[100000] = { 0 };
char fire = 0;
int pattara = 0;

int sub(int c) {

	
	
	/*while(i>-1)*/
	for (int i = 2; i<=c; i++){
		/*for (int z = 0; z < a;z++){*/
		for (int q = 1; q <= sqrt(i); q++) {
			sosuu[pattara] = i;
			if (sosuu[pattara] % 2 == 0 && sosuu[pattara]!= 2) {
				break;
			}
			if (sosuu[pattara] % q == 0) {
				b += 1;
			}

		}

		if (b == 1) {
			/*fire = _getch();*/
			pattara++;
			
		}
		b = 0;
		/*i++;*/
	}

	
	return 0;
}


int main(){
	while (scanf("%d", &c) != EOF){

		sub(c);
		cout << pattara << endl;
		pattara = 0;
		
	}

	return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>
#include <string>
#include <cstring>
#include <cctype>
#include <cmath>
#include <stack>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <list>
#include <stdio.h>
#include <string.h>
#include <cstdlib>
#include <math.h>
#include <bitset>
#include <iterator>
#include <iomanip>
#include <sstream>
#include <numeric>
#define INF 2000000000
using namespace std;
int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
long long gcd(long long a,long long b){if(a%b==0){return b;}else return gcd(b,a%b);}
long long lcm(long long a,long long b){if(a==0){return b;} return a/gcd(a,b)*b;}
long long MOD; template<class T> void mod(T &n){MOD=n;}
#define sym cout<<"---------"<<endl;
#define ll long long
#define PL pair<long long, long long>
#define P pair<int,int>
#define mk make_pair
#define en endl
#define WHITE 1
#define BLACK 2
#define GRAY 1
#define RE return 0
#define int ll

bool isprime(int n){
  if(n==2||n==3) return true;
  for(int i=2; i*i<=n; i++) if(n%i==0) return false;
  
  return true;
}

bool prime[1000005]={true};

signed main(){
  int n;
  while(cin>>n){
    if(n==1){cout<<0<<en; RE;}
    for(int i=2; i<=n; i++) prime[i]=true;
    int ans=0;
    for(int i=2; i<=n; i++){
      if(isprime(i)&&prime[i]) {
        ans++;
        for(int j=2*i; j<=n; j+=i) prime[j]=false;
      }
    }
    cout<<ans<<en;
  }
}

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

int prime[1000010];
bool is_prime[1000010];

void seive(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;
		}
	}
}
	
int main() {
	int n;
	
	seive(999999);
	
	while(cin>>n){
		int sum=0;
		
		for(int i=0;i<=n;i++)
		if(is_prime[i])
		sum++;
		
		cout<<sum<<endl;
	}
	return 0;
}

C++: #include<iostream>
#include<algorithm>
using namespace std;
/*
int main(){
	
	int i,j;
	bool n[30]={false};
	for(j=0;j<28;j++){
	cin>>n[j];
	n[j]=true;
	}
	for(i=0;i<30;i++){
		if(n[i]==false){
		cout<<i<<endl;
		}
	}

return 0;
}
*/
int main(){
	int a[30],b[28];
	for(int i=1;i<=30;i++){
		a[i-1]=i;
	}
	for(int i=0;i<28;i++){
		cin>>b[i];
	}
	sort(b,b+28);
	int c[2]={0},j=-1,k=-1;
	for(int i=0;i<28;i++){
		j++;
		if(a[i]!=b[j]){
			k++;
			c[k]=a[i];
			j--;
		}
		if(j==27){
			break;
		}
	}
	if(c[0]==0){
		c[0]=29;
	}
	if(c[1]==0){
		c[1]=30;
	}
	cout<<c[0]<<endl<<c[1]<<endl;
	return 0;
}

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

int main(){
	int n,w=0;
	cin >> n;
	for(int j=n;j>1;j--){
		for(int i=2;i<j;++i){
			if(j%i == 0){
				break;
			}
			if(i==j-1){
				w++;
			}
		}
	}
	cout << w;	
	return 0;
}

C++: //============================================================================
// Name        : desk.cpp
// Author      :
// Version     :
// Copyright   : Your copyright notice
// Description : Hello World in C++, Ansi-style
//============================================================================

#include<stdio.h>
#include<iostream>
using namespace std;
int main(){

 int n=0,f=0,P=0;

 while(scanf("%d",&n)!=EOF){

 for(int i=2;i<n;i++){

	 if(n%i==0){
		f=1;
		}

 }

 if(f==0){
	 P++;
 }
 
 }
 cout<<P<<endl;
  return 0;
}

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

int main(){
	int a;
	while(cin>>a){
		int ans = 1;
		for(int i = 2; i <= a/i; i++){
			ans += i + a/i;
			if(a == i*i) ans -= i;
		}
		if(ans == 1) cout << a << endl;
	}
	return 0;
}

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

bool check(int in){
	for (int i = 3; i<=sqrt(in); i+=2){
		if (i != 3 && i % 3 == 0)	continue;
		if (i != 5 && i % 5 == 0)	continue;
		if (i != 7 && i % 7 == 0)	continue;
		if (in % i == 0)
			return false;
	}
	return true;
}

int PrimeNumber(int in, int start){
	int count = 0;
	if (in > 2)	count++;
	for (int i = start; i <=in; i+=2){
		if (check(i))	count++;
	}

	return count;
}

int main(){
	int in, count500000 = PrimeNumber(500000, 3);

	while (cin >> in, !cin.eof()){
		if (in <= 500000)
			cout << PrimeNumber(in, 3) << endl;
		else
			cout << count500000 + PrimeNumber(in, 500001) << endl;
	}
}

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

using namespace std;

int prime(int n) {
  int i, j, r;
  
  if (n < 2) return 0;
  vector<int> primes(n);

  for (i = 2; i < n; i++)
    primes[i] = i;
  for (i = 2; i*i < n; i++) {
    if (primes[i])
      for (j = i*i; j < n; j += i)
        primes[j] = 0;
  }
  for (i = 2, r = 0; i < n; i++)
    if (primes[i]) r++;
  return r;
}

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

C++: #include <iostream>
#include <stdio.h>
#include <math.h>
#include <vector>
#include <algorithm>
using namespace std;

int check(int x,int y,int z,int w){

}

int main(){
	int n;
	while(cin >> n){
		vector<int> prim;
		prim.push_back(2);
		for(int i=3;i<=n;i++){
			bool fl=false;
			for(int j=0;j<prim.size();j++){
				if(i%prim[j]==0){
					fl = true;
					break;
				}
			}
			if(fl==false) prim.push_back(i);
		}
		cout << prim.size() << endl;
	}
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <stack>
#include <string>
#include <map>
#include <functional>
#include <cmath>
#include <cstdio>
using namespace std;

#define PI 4*atan(1);

vector<int> a;
void func(int n){

  for(int i = 0; i <= n; i++){
    a.push_back(1);
  }
  for(int i = 2; i * i <= n; i++){
    if(a[i]){
      for(int j = 2; i * j <= n; j++){
        a[i * j] = 0;
      }
    }
  }
  int cnt = 0;
  for(int i = 2; i <= n; i++){
    if(a[i]){
      cnt++;
    }
  }
  cout << cnt << endl;
}

int main(){
  int n;
  while(cin >> n){
    func(n);
  }
}