Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextInt() int {
	i, e := strconv.Atoi(scanner.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func check(n int) int {
	eratos := make([]bool, n+1)
	cnt := 0
	for i := 2; i < len(eratos); i++ {
		if !eratos[i] {
			cnt++
			for j := i*i; j < len(eratos); j += i {
				eratos[j] = true
			}
		}
	}
	return cnt
}

func main() {
	for scanner.Scan() {
		n := nextInt()
		fmt.Println(check(n))
	}
}

C++: #include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < (int)(n); i++)

using namespace std;

typedef long long ll;
typedef pair<int, int> P;
typedef pair<ll, ll> LP;

const int MOD = 1000000007;
const int INF = 1000000000;
const ll LINF = 1000000000000000000L;
const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};
const int ddx[] = {1, 1, 0, -1, -1, -1, 0, 1};
const int ddy[] = {0, 1, 1, 1, 0, -1, -1, -1};

int extgcd(int a, int b, int& x, int&y) {
    int d = a;
    if(b != 0) {
        d = extgcd(b, a % b, y, x);
        y -= (a / b) * x;
    }else {
        x = 1;
        y = 0;
    }
    return d;
}

int inv(int a) {
    int x, y;
    extgcd(a, MOD, x, y);
    return (x + MOD) % MOD;
}

const int MAX_N = 1000000;
bool ifprime[MAX_N + 1];
bool used[MAX_N + 1];
int sum [MAX_N + 1];

int main() {
    fill(used, used + MAX_N + 1, false);
    fill(ifprime, ifprime + MAX_N + 1, false);
    for(int i = 2; i <= MAX_N; i++) {
        if(!used[i]) {
            used[i] = true;
            ifprime[i] = true;
            for(int j = 1; i * j <= MAX_N; j++) {
                used[i * j] = true;
            }
        }
    }

    sum[0] = 0;
    rep(i, MAX_N) {
        sum[i + 1] = sum[i];
        if(ifprime[i + 1]) sum[i + 1]++;
    }

    int n;
    while(cin >> n) {
        cout << sum[n] << endl;
    }
}

C++: #include<iostream>
#include<vector>
using namespace std;
 int main(){
    int n,cnt;
  while(cin >> n){
    cnt=0;
    vector<bool>m(n+1,true);
    for(int i=2;i*i<=n;i++)
         if(m[i]) for(int j=i+i;j<=n;j+=i)  m[j]=false;        
    for(int i=2;i<=n;i++)
            if(m[i]) cnt++;
    cout << cnt << endl;  
  } 
  return 0;
}

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

using namespace std;

int get_numprime( int num );

int main(void)
{
	int num;

	while(1)
	{
		cin >> num;
		if( cin.eof() ) break;
		cout << get_numprime( num ) << endl;
	}

	return 0;
}

int get_numprime( int num )
{
	int numprime = 1; // 2はすでにカウント

	for( int i = 3 ; i <= num ; i+=2 )
	{
		int flag = 1;
		for( int j = 3 ; j <= sqrt((double)i) ; j+=2 )
		{
			if( i%j == 0 )
			{
				flag = 0;
				break;
			}
		}
		if( flag == 1 ) numprime++;
	}

	return numprime;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int prime(int);
int main(){
    int a,n;
    n=0;
    while ( cin >> a){
        for (int i=2;i<a;i++){
            if (prime(i)==0){
                n++;
            }
        }
        cout << n << endl;
    }
}


int prime(int x){
    if(x==2 || x==3 || x==5 || x==7)
        return 0;
    if(x%2==0 || x%3==0)
        return 1;
    for (int i=5;i<=sqrt(x)+1;i+=6){
        if(x%i==0 || x%(i+2)==0)
            return 1;
    }
    return 0;
}

C++: #include <iostream>
#include <string>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <utility>
#include <iomanip>

#define ll long long int
#define pb push_back
#define mk make_pair
#define pq priority_queue

using namespace std;
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
const int inf = 1e9;
const ll linf = 1LL << 50;

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

int main(int argc, char const* argv[])
{
	fill(is_prime, is_prime+1000000, true);
	is_prime[0] = false;
	is_prime[1] = false;
	int res = 0;
	for(int i = 2; i < 1000000; i++){
			if(is_prime[i]){
					res++;
					for(int j = 2; j * i < 1000000; j++){
							is_prime[j * i] = false;
					}
			}
			prime[i] = res;
	}
	while(cin >> n){
			cout << prime[n] << endl;
	}
	return 0;
}

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

using namespace std;
int main() {
  long n;
  vector<long> primes;
  primes.push_back(2);
  while (cin >> n) {
    if (n == 1) {
      cout << 0 << endl;
      return 0;
    }
    for (int i = 3; i <= n; i += 2) {
      bool prime = true;
      for (unsigned int j = 0; j < primes.size() && primes[j] * primes[j] <= n; j++)
        if (i % primes[j] == 0) {
          prime = false;
          break;
        }
      if(prime && find(primes.begin(), primes.end(), i) == primes.end()) {
        primes.push_back(i);
      }
    }
    int x = 0;
    for (long& e: primes)
      if (e <= n) x++;
    cout << x << endl;
  }
}

C++: #include<Upstream>
#include<cstdio>
#include<cstring>
using namespace std;

Int pn[1000002];

int main(){
int n, Count;
con.tie(0);
while(scanf("%d",&n)!=EOF){
Count=0;
memset(pn,0,n);
pn[0]=pn[1]=1;
for(int i=0;i<=n;i++) if(!pn[i]) for(int j=0;i*j<=n;j++) pn[i*j]=1;
Count=count(pn,pn+n,0);
count<<Count<<endl;
}
return 0;
}

C++: #include <iostream>
#include<string>
#include<cmath>
#include<algorithm>
#include<cctype>
#include<queue>
#include<stack>
#include<stdio.h>
#include<vector>
#include<set>
#include<map>
#include<iomanip>
#define rep(i,n) for(int i=0;i<n;i++)
typedef int long long ll;

using namespace std;
/*int main(){
    
    ll n;
    while (cin>>n) {
        vector<ll> v;
        for(int i=0;i<n+1;i++){
            v.push_back(i);
        }
        ll b=v[v.size()-1];
        for(int i=2;i<=n;i++){
            ll j=2;
            while(i*j<=b){
                v[i*j]=0;
                j++;
            }
        }
        ll ans=0;
        for(int i=0;i<v.size();i++){
           // cout<<v[i]<<endl;
            if(v[i]!=0)ans++;
        }
        cout<<ans-1<<endl;
    }
    
    
    return 0;
}*/
int main(){
    
    ll n;
    while(cin>>n){
       /* ll ans=0;
        for(int i=3;i<=n;i+=2){
            int flag=0;
        for(int j=3;j<=sqrt(i);j+=2){
            if(i%j==0){
                flag=1;
                break;
            }
        }
            if(flag==0){
                ans++;
            }
    }
        cout<<ans+1<<endl;*/
        
        vector<bool> IsPrime;
        if(n+1>IsPrime.size()){
            IsPrime.resize(n+1,true);
        }
        IsPrime[0]=false;
        IsPrime[1]=false;
        for(int i=2;i<=sqrt(n);i++){
            if(IsPrime[i]){
                for(ll j=2;i*j<=n;++j){
                    IsPrime[i*j]=false;
                }
        }
        
        }
        ll ans=0;
        for(int i=0;i<IsPrime.size();i++){
            if(IsPrime[i])ans++;
        }
        cout<<ans<<endl;
        }
    
    return 0;
}

C++: #include<iostream>
#include<list>
#include <vector>
#include <numeric>
#include <algorithm> 
using namespace std;

int main(){
	int N;
	int r;
	int j;
	int count;
	list<int> counter;
	
	while(!(cin>>N).eof()){
		if(N==2 ){
			counter.push_back(1);
		}
		else if(N==3){
			counter.push_back(2);
		}
		else {
			count=0;
			for(int i=3; i<=N; i+=2){
				j=3;
				do{
					r = i%j;
					if(j!=3 && r == 0) break;
					j+=2;
				}while(j<=(i/2)+1);
				if(i!=3 && r==0) continue;
				count++;
				
			}
			counter.push_back(count);
		}	
	}
	list<int>::iterator it = counter.begin();
	while(it != counter.end()){
		cout << *it << endl;
		it++;
	}
	return 0;
}