Go: package main

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

func search(min, max int, channel chan int) {
	flag := false
	count := 0
	for i := min; i <= max; i++ { //1を除く入力値以下の数字
		for j := 2; j < i; j++ {
			if i%j == 0 {
				flag = true
				break
			}
		}
		if !flag {
			count++
		} else {
			flag = false
		}
	}
	channel <- count
	return
}

func main() {
	count := 0
	channel := make(chan int)
	scanner := bufio.NewScanner(os.Stdin)
	for scanner.Scan() {
		num, _ := strconv.Atoi(scanner.Text())
		for i := 2; i < num; i += 10 {
			if i+10 < num {
				count++
				go search(i, i+10, channel)
			} else {
				count++
				go search(i, num, channel)
			}
		}
		var ans int
		for i := 0; i < count; i++ {
			ans += <-channel
		}

		fmt.Println(ans)
	}
}

C++: #include<iostream>
using namespace std;
int main(){
  int a,t[1000000],u[1000000];
  for(int i=0;i<1000000;i++){
    t[i]=1;
  }
  t[0]=t[1]=u[0]=u[1]=0;
  for(int i=2;i<=1000000;i++){
    if(t[i]==1){
      for(int j=i+i;j<1000000;j+=i)t[j]=0;
      u[i]=u[i-1]+1;
    }else{
      u[i]=u[i-1];
    }
  }
  while(cin>>a){
    cout<<u[a]<<endl;
  }
  return 0;
}

C++: // 6/2
// 2-6-4. 素数のこすう
// エラトステネスの篩
#include <bits/stdc++.h>
#define rep(i, n) for (ll i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
using P = pair<int, int>;
const int INF = 1e9;

// const int MAX_N = 1000000;

vector<int> prime;  // i番目の素数
vector<bool> is_prime;  // iが素数かどうか

void sieve(int n) {
    // 一旦全ての数字を素数に。
    prime.resize(n, 0);
    is_prime.resize(n + 1, true);

    // 0, 1は素数では無い。
    is_prime[0] = is_prime[1] = false;

    int p = 0;

    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() {
    sieve(1e6);

    int n;
    vector<int> ans;

    while (cin >> n) {
        int an = 0;
        rep(i, n + 1) {
            if (is_prime[i]) an++;
        }
        ans.push_back(an);
    }

    for (auto a : ans) cout << a << endl;
}

C++: #include <iostream>

using namespace std;

int main()
{
    int n;
    int prime[100000];
    long long ptr = 0;

    prime[ptr++] = 2;
    prime[ptr++] = 3;

    while (cin >> n) {
        if (n <= 1) {
            cout << "0\n";
            continue;
        }
        else if (n == 2) {
            cout << "1\n";
            continue;
        }
        else if (n == 3) {
            cout << "2\n";
            continue;
        }

        int count = 2;        
        for (int i = 5; i <= n; i += 2) {
            bool flag = false;
            for (int j = 1; prime[j] * prime[j] <= i; j++) {
                if (!(i % prime[j])) {
                    flag = true;
                    break;
                }
            }
            if (!flag) {
                prime[ptr++] = i;
                count++;
            }
        }
        cout << count << "\n";
    }
    
    return 0;
}

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

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

int main(void)
{
	int number = 0;
	int count = 0;

	while(cin >>  number){
		count = 0;
		for(int i=2;i<=number;i++){
			for(int j= 2 ;j<=i/2;j++){
				if(i % j == 0){//割り切れる＝素数じゃない
					goto go;
				}
			}
			count++;
go:
		;
		}
		cout << count << endl;
	}
	return 0;
}

C++: #include<stdio.h>

int prime(int n);
int main(){
	int count=0;
	int n;
	while(scanf("%d",&n)!=EOF){
		printf("%d\n",prime(n));
	}
	return 0;
}

int prime(int n){
	int i,j;
	int p=0;
	int count=0;
	for(i=2;i<=n;i++){
		for(j=2;j<i;j++){
			if(i%j==0)p++;
		}
		if(p==0)count++;
		p=0;
	}
	return count;
}

C++: #include<stdio.h>
#include<math.h>
int isprime(int x){
	int i=3;
	while( i <= sqrt(x) ){
		if(x%i==0)return 0;
		i = i + 2;
	}
	return 1;
}
int main(){
	int n,c,i;
	while(scanf("%d",&n)!=EOF){
		c=1;
		for(i=3;i<=n;i+=2)if(isprime(i))c++;
		printf("%d\n",c);
	}
	return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <deque>
#include <list>
#include <string>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <algorithm>

using namespace std;

int n;
bool prime[1000000];

int main() {

	for (int i = 2; i <= 1000000; i++) prime[i] = true;

	for (int i = 2; i <= 1000000; i++) {
		if (!prime[i]) continue;
		for (int j = 2; i * j <= 1000000; j++) {
			prime[i * j] = false;
		}
	}

	n = -1;
	cin >> n;

	while (n != -1) {
		int cnt = 0;
		for (int i = 2; i <= n; i++) {
			if (prime[i]) cnt++;
		}
		cout << cnt << endl;

		n = -1;
		cin >> n;
	}

	return 0;
}

/*@@@@@@@@@@@@@@@@@@@@@@@@@*/
/*@                       @*/
/*@ debug output erased ? @*/
/*@                       @*/
/*@@@@@@@@@@@@@@@@@@@@@@@@@*/

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

int main() {
	int n, c;

	while (std::cin >> n) {
		std::vector<int> nums;
		++n;
		c = 0;
		for (int i = 0; i < n; ++i) {
			nums.push_back(i);
		}
		for (int i = 0; i < n; ++i) {
			if (nums.at(i) < 2) continue;
			for (int j = nums.at(i) * 2; j < n; j += nums.at(i)) {
				nums.at(j) = 0;
			}
		}
		for (int i = 0; i < n; ++i) {
			if (nums.at(i) > 1) {
				++c;
			}
		}
		std::cout << c << std::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<stdio.h>
#include<float.h>
#include<vector>



int makePrimenumbers(int a){
	int temp;
	
	if(a<ref_max)return 0;

	temp = ref_max;
	ref_max=a;
	int i,k,isp=1;

	for(k=temp;k<ref_max;k++){
		for(i=0;i<primes.size();i++){
			if(k%primes[i]==0){
				isp=0;
				break;
			}
		}
		if(isp==1){
			primes.push_back(k);
		}
		isp=1;
	}
	return 0;
}

int countprime(int a){
	int i;
	for(i=0;i<prime.size();i++)
		if(a<prime[i])
			break;
	return (i);
}

int main(void)
{
	vector<int> primes;
	int ref_max=2;
	int result;
	int a;
	while(scanf("%d",&a)!=EOF){
		makePrimenumbers(a);
		result = countprime(a);
		printf("%d\n",result);
	}
  return 0;
}