Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	var primeCount int
	for sc.Scan() {
		input := nextInt()
		for i := 0; i <= input; i++ {
			if IsPrime(i) {
				primeCount++
			}
		}
		fmt.Printf("%d\n", primeCount)
		primeCount = 0
	}
}

func IsPrime(num int) bool {
	switch {
	case num == 2:
		return true
	case num < 2 || num%2 == 0:
		return false
	}
	sqrt := math.Sqrt(float64(num))
	for i := 3; i <= int(sqrt); i += 2 {
		if num%i == 0 {
			return false
		}
	}
	return true
}

func next() string {
	return sc.Text()
}

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

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

bool primes[1000000];

void Sieve(){
    for(int i= 2; i < 1000000; i++){
        primes[i] = true;
    }
    primes[1] = false;    
    for(int i = 2; i < 1000; i++){
        if(primes[i] == true)
            for(int j=i; j*i < 1000000;j++){
                primes[j*i] = false;
            }
    }     
}
bool IsPrime(int x){
    return primes[x];
}

int main(void){
    Sieve();
    int n, c;
    while(cin >> n){
        c = 0;
        for(int i = 2; i <= n; i++){
        if(IsPrime(i)==true)
            c++;
        }
        cout << c << endl;
    }
    return 0;
}

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

using namespace std;

#define MAX_VALUE 1000000

int get_numprime( int num );

int main(void)
{
	int num;

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

	return 0;
}

int get_numprime( int num )
{
	int numprime = 1; // 2はカウント済み

	bool erat[MAX_VALUE] = {0};

	for( int i = 0 ; i < num+1 ; i++ )
	{
		erat[i] = true;
	}
	erat[0] = false; erat[1] = false;
	for( int i = 2 ; i <= num ; i++ )
	{
		if( erat[i] )
		{
			for( int j = i*2 ; j <= num ; j+=i )
			{
				erat[j] = false;
			}
		}
	}
	for( int i = 3 ; i <= num ; i+=2 )
	{
		if( erat[i] ) numprime++;
	}

	if( num == 1 ) numprime = 0;

	return numprime;
}

C++: #include <bitset>
#include <iostream>
#include <string>

using namespace std;

int main()
{
    bitset<999999 + 1> b;
    auto t = b;
    t.set();
    t[0] = false;
    t[1] = false;
    int i = 0;
    while (true) {
        bool done = false;
        while (!t[i]) {
            ++i;
            if (i == t.size()) {
                done = true;
                break;
            }
        }
        if (done) {
            break;
        }

        b[i] = true;
        for (int j = i; j < t.size(); j += i) {
            t[j] = false;
        }
    }

    string s;
    while (getline(cin, s)) {
        int n = stoi(s);
        int count = 0;
        for (int j = 1; j <= n; ++j) {
            if (b[j]) {
                ++count;
            }
        }
        cout << count << endl;
    }
}

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<bool> ip;
        if(ip.size()<n+1){
            ip.resize(n+1,true);
        }
        ip[0]=false;
        ip[1]=false;
        for(int i=2;i<=(n+1)/2;i++){
            for(int j=2;i*j<n+1;j++){
                ip[i*j]=false;
            }
        }
        ll ans=0;
        for(int i=0;i<n+1;i++){
            if(ip[i])ans++;
        }
        
        
        cout<<ans<<endl;
        
        
        
    }





return 0;
}

C++: #include <stdio.h>
#include <math.h>
int check_prime(int a)
{
   int c,b;
   b = (int)sqrt(a);
   for ( c = 2 ; c <=  b; c++ )
   {
      if ( a%c == 0 )
        return 0;
   }
      return 1;
}

int main ()
{
    int  n=0,i=0,prime=0,result=0;

   while( scanf("%d",&n) != EOF)
{
 for(i=2;i<=n;i++)
 {

   result = check_prime(i);

   if ( result == 1 )
     prime = prime +1;
 }
 printf("%d",prime);
 printf("\n");
 prime=0;
}
 return 0;
}

C++: #include<stdio.h>
int main(){
	int a,b,n;
	int p[80000];
	p[0]=2;
	n=1;
	for(a=3;a<1000000;a++){
		for(b=0;b<n;b++){
			if(a%p[b]==0){break;}
			if(a<p[b]*p[b]){
				p[n]=a;
				n++;
				break;
			}
		}
	}
		scanf("%d",&a);
		n=0;
		while(a>=p[n]){
			n++;
		}
		if(a<2){n=0;}
		printf("%d\n",n);
	return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
bool b[1000000];
int n,i,j,ans;
int GCD(int x,int y)
{
	return y==0?x:GCD(y,x%y);
}
void biao()
{
	for(i=2;i<=999999;i++)
	{
		if(b[i]==0)
		for(j=i+i;j<=999999;j=j+i)
			b[j]=1;
	}
}
int main()
{
	biao();
	while(~scanf("%d",&n))
	{
		ans=0;
		for(i=2;i<=n&&i<5;i++)
		{
			if(b[i]==0)
				ans++;
		}
		for(i=5;i<=n;i=i+6)
		{
			if(b[i]==0)
				ans++;
			if(b[i+2]==0&&i+2<=n)
				ans++;
		}
		printf("%d\n",ans);
	}
}

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;
}