Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

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

func isPrime(n int) bool {
	for i := 2; i*i <= n; i++ {
		if n % i == 0 {
			return false
		}
	}
	return true
}

func main() {
	for scanner.Scan() {
		n := nextInt()
		cnt := 0
		for i := 2; i <= n; i++ {
			if isPrime(i) {
				cnt++
			}
		}
		fmt.Println(cnt)
	}
}

C++: #include <iostream>
#include <stdio.h>
#include <algorithm>
#include <functional>

using namespace std;

#define MAXN 1000016

int isprime[MAXN];

void sieve(int n)
{
	int i, j, num, p = 0;
	fill(isprime, isprime + MAXN + 1, 1);
	for (i = 2, num = 1; i <= n; i++)
	{
		if (isprime[i])
		{
			isprime[i] = num++;
			for (j = 2 * i; j <= n; j += i)
				isprime[j] = 0;
		}
	}
}

int main()
{
	int n;
	sieve(MAXN);
	while (scanf("%d", &n) != EOF)
	{
		while (!isprime[n])
		{
			n--;
		}
		printf("%d\n", isprime[n]);
	}
	return 0;
}

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

int main(){
	int n;
	while(cin >> n){
		int counter = 0;
		if(n != 1)counter++;
		for(int i = 3; i <= n; i += 2){
			for(int j = 2; j <= sqrt(i); j++){
				if(i % j == 0)goto divisible;
			}
			counter ++;
			divisible:;
		}
		cout << counter << endl;
	}
}

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

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

using namespace std;

bool prime[(int)1e7];
int integral[(int)1e7];

int main(){
	fill(prime + 2, prime + (int)1e7, true);
	
	for(int i = 2; i < (int)1e7; ++i){
		if(prime[i]){
			for(int j = i * 2; j < (int)1e7; j += i){
				prime[j] = false;
			}
		}
	}

	for(int i = 1; i < (int)1e7; ++i){
		integral[i] = integral[i - 1] + prime[i];
	}

	int n;
	while(scanf("%d", &n) != EOF){
		printf("%d\n", integral[n]);
	}
	return 0;
}

C++: #include <stdio.h>
#include <iostream>

using namespace std;

#define MAX_NUM	1000000

int solist[MAX_NUM] = { 0 };

int main(int argc, char* argv[])
{
	int i; 
	int num;
	int count;
	int val;

	solist[0] = 1;
	solist[1] = 1;

	for (num = 2; num < MAX_NUM; num++) {
		if (solist[num]) continue;
		for (i = num * 2; i < MAX_NUM; i += num) {
			solist[i] = 1;
		}
	}

	while (cin >> val) {
		count = 0;
		for (i = 1; i <= val; i++) {
			if (solist[i] == 0) {
				count++;
			}
		}
		cout << count << endl;
	}
	
	return 0;
}

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

 void prima()
    {
        bool isprima;
        int a, n;
        while(cin >> a)
        {
            n = 0;
            for(int i = 2; i <= a; i++)
            {
                isprima = true;
                for(int j = 2; j < i; j++)
                {
                    if( i == 2)
                    {
                        isprima = true;
                        break;
                    }
                    else if(i % j == 0)
                    {
                        isprima = false;
                        break;
                    }
                }
                if(isprima) n++;
            }
            cout << n << endl;
        }
    }

int main()
{
   prima();
}

C++: #include<iostream>
#include<cmath>
using namespace std;
int main()
{
	int a[999999];
	a[0] = 2;
	long long k = 1;
	for (long long i = 2; i<=999999; i++)
	{
		int flag = 1;
		for (long long j = 2; j <= sqrt(i) + 1;j++)
		{
			if (i%j == 0)
			{
				flag = 0;
				break;
			}
		}
		if (flag)
			a[k] = i, k ++ ;
	}
	int n;
	while (cin >> n)
	{
		long long ans = 0;
		for (long long x = 0; x < k; x++)
		{
			if (n >= a[x])
				ans++;
			else
				break;
		}
		cout << ans << endl;
	}
	return 0;
}

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

using namespace std;

void Eratosthenes(vector<int> &S_List, vector<int> &P_List){
	while(1){
		vector<int>::iterator it = S_List.begin();
		P_List.push_back(*it);  //先頭を素数リストへ追加
		int num = *it;
		while (it != S_List.end()){
			if ((*it % num) == 0){
				it = S_List.erase(it); //探索リストから倍数を除外
			}
			else{
				++it;
			}
		}
		vector<int>::iterator P_it = max_element(P_List.begin(), P_List.end());
		double p_max_sqr = sqrt(*P_it);
		vector<int>::iterator S_it = max_element(S_List.begin(), S_List.end());
		if (S_List.empty() == true){
			return;
		}
		if (*S_it < p_max_sqr){
			return;
		}
		/*else{
			Eratosthenes(S_List, P_List);
		}*/
	}
}


void output(vector<int> ex){
	/*vector<int>::iterator it = ex.begin();
	while(it != ex.end()){
		cout << *it << endl;
		++it;
	}*/
	cout << ex.size() << endl;
}

int main(void){
	int num;
	while (cin >> num){
		vector<int> S_List, P_List;
		for (int i=2; i<=num; ++i){
			S_List.push_back(i);
		} 
		Eratosthenes(S_List, P_List);
		output(P_List);
	}
	return 0;
}

C++: #include <iostream>
#define N 100000
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;

int main(void)
{
    bool a[N+1];
    int prime[N], n;
    rep(i,N)
    {
        a[i] = true;
    }
    a[0] = a[1] = false;
    for(int i=2;i*i<=N;i++)
    {
        if(a[i])
        {
            prime[i] = prime[i-1] + 1;
            for(int j=i*i;j<N;j+=i)
                a[j] = false;
        }
        else
            prime[i] = prime[i-1];
    }

    while(cin >> n)
        cout << prime[n] << endl;
    
    return 0;
}

C++: #include<bits/stdc++.h>
#define rep(i,n) for(int i=0; i<n; i++)
using namespace std;
int main(){
  int n;
  while(cin >> n){
    int cnt=0,res=0;
    for(int i=2; i<=n; i++){
      for(int j=1; j<=n; j++){
        if(i%j == 0) cnt++;
        if(cnt > 2) break;
      }
      if(cnt==2) res++;
      cnt = 0;
    }
    cout << res << endl;
  }
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
inline bool prime(int x){
	for(int i=2;i<x;i++)
		if(!(x%i))	return false;
	return true;
}
int main(){
	int n;
	while(cin>>n){
		int ans=0;
		for(int i=2;i<=n;i++)
			if(prime(i))	ans++;
		cout<<ans<<endl;
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
int main()
{
	int n,S;
while(cin>>n)
{
	S=0;
	if(n==1)
	{
		cout<<0<<endl;
	}
	for(int i=2;i<=n;i++)
	{
		for(int j=i-1;j>1;j--)
		{
			if(i%j==0)
			{
				break;
			}
			if(j==2)
		{
			S++;
		}
		}	
			
	}
	cout<<S<<endl;
}
return 0;
}

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

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

C++: //{Headers
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<ctime>
#include<iostream>
#include<string>
#include<algorithm>
#include<list>
#include<queue>
#include<deque>
#include<vector>
#include<stack>
#include<map>
#include<set>
#include<bitset>

using namespace std;
typedef vector< int > vi;
typedef vector< vi > vvi;
typedef pair< int,int > pii;
#define mp(x,y) make_pair(x,y)
typedef long long ll;
//}

const int MAXN=1e6;
int N,nPrime[MAXN];

bool isPrime[MAXN];
void EratosthenesSeive(){
    memset(isPrime,true,sizeof isPrime);
    isPrime[0]=isPrime[1]=false;
    for(int i=2;i<MAXN;i++) if(isPrime[i]){
        for(int j=i<<1;j<MAXN;j+=i)
            isPrime[j]=false;
    }
}

void init(){
    EratosthenesSeive();
    nPrime[0]=0;
    for(int i=1;i<MAXN;i++) nPrime[i]=nPrime[i-1]+(isPrime[i]?1:0);
}

void solve(){printf("%d\n",nPrime[N]);}

int main(void){
//    freopen("in.txt","r",stdin);
    init();
    while(~scanf("%d",&N))
        solve();
    return 0;
}

/*
Sample Input
10
3
11
Output for the Sample Input
4
2
5
*/