Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

C++: //#define _GRIBCXX_DEBUG
#include <algorithm>
#include <cctype>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iostream>
#include <iomanip>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
using namespace std;


// ?????¬???????????¬??????
#pragma region MACRO
#define P(x) cout << (x) << endl
#define p(x) cout << (x)
#define PED cout << "\n"
#define rep(i,n) for(int i=0; i<(int)n; ++i)
#define REP(i,x,n) for(int i=x; i<(int)n; ++i)
#define repi(i,n) for(int i=0; i<=(int)n; ++i)
#define REPI(i,x,n) for(int i=x; i<=(int)n; ++i)
#define ILP while(true)
#define FOR(i,c) for(__typeof((c).begin())!=(c).begin(); i!=(c).end(); ++i)
#define ALL(c) (c).begin(), (c).end()
#define mp make_pair
#pragma endregion

#pragma region TYPE_DEF
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<string, string> pss;
typedef pair<string, int> psi;
typedef pair<int, string> pis;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<long> vl;
typedef vector<long long> vll;
typedef vector<string> vs;
#pragma endregion


// Effective std
#pragma region ESTD
template<typename C, typename T>
constexpr int count(C& c, T t) { return count(ALL(c), t); }
template<typename C, typename F>
constexpr int count_if(C& c, F f) { return count_if(ALL(c), f); }
template<typename C, typename T, typename U>
constexpr void replace(C& c, T t, U u) { replace(ALL(c), t, u); }
template<typename C, typename F, typename U>
constexpr void replace_if(C& c, F f, U u) { (ALL(c), f, u); }
template<typename C>
constexpr void sort(C& c) { sort(ALL(c)); }
template<typename C, typename Pred>
constexpr void sort(C& c, Pred p) { sort(ALL(c), p); }
template<typename C>
constexpr void reverse(C& c) { reverse(ALL(c)); }
#pragma endregion


// ?´???°
#pragma region PRIME
bool is_prime(unsigned n) {
  switch(n) {
    case 0:
    case 1: return false;
    case 2: return true;
  }
  if (n%2==0) return false;
  for (unsigned i=3; i*i<=n; i+=2)
    if (n%i==0) return false;
  return true;
}
#pragma endregion


// ??§??????/?°?????????????
#pragma region TRANSFORM
void mutal_tr(string &s) {
  for(int i=s.size(); i--;) {
    if(islower(s[i])) s[i] = toupper(s[i]);
    else if (isupper(s[i])) s[i] = tolower(s[i]);
  }
}
void to_upper(string &s) { for(int i=s.size(); i--;) s[i] = toupper(s[i]); }
void to_lower(string &s) { for(int i=s.size(); i--;) s[i] = tolower(s[i]); }
#pragma endregion


// ??????
#pragma region SET
template<class T>
set<T> intersection(const set<T>& sa, const set<T>& sb) {
  set<T> ret;
  for(T a : sa) if(sb.find(a) != sb.end()) ret.insert(a);
  return ret;
}
#pragma endregion

// Union Find
#pragma region UF
struct UnionFind {
  vector<int> data;
  UnionFind(int size) : data(size, -1) {}
  bool union_set(int x, int y) {
    x = root(x); y = root(y);
    if(x!=y) {
      if(data[y] < data[x]) swap(x, y);
      data[x] += data[y]; data[y] = x;
    }
    return x != y;
  }
  bool find_set(int x, int y) { return root(x) == root(y); }
  int root(int x) { return data[x] < 0 ? x : data[x] = root(data[x]); }
  int size(int x) { return -data[root(x)]; }
};
#pragma endregion


// ?????°
#pragma region CONST_VAL
#define PI (2*acos(0.0))
#define EPS (1e-9)
#define MOD (int)(1e9+7)
#pragma endregion

int main()
{
    ll n;
    while(cin>>n) {
        int ans = 0;
        REPI(i,2,n) {
            if(is_prime(i)) ans++;
        }
        P(ans);
    }
    return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
  int n;
  int i,j,c,p;

  while(cin>>n){
    c=1;
    for(i=3;i<=n;i+=2){
      p=0;
      for(j=2;j*j<=i;j+=2){
	if(n%i==0){
	  p=1;
	  break;
	}
      }
      if(p==0){
	c++;
      }
    }
    cout<<c<<endl;
  }
  return 0;
}

C++: #include <iostream>
#include <math.h>
using namespace std;
int main(){
	int n;
	while(cin>>n){
		int prime;
		if(n>3){
			prime=2;
			for(int i=5;i<=n;){
				int m=sqrt((double)i);
				for(int j=3;j<=m+1;j++){
					if(i%j==0)
						break;
					else{
						if(j==m+1) prime++;
					}
				}
				i += 2;
			}
		}
		else if(n==2) prime=1;
		else if(n==3) prime=2;
		cout<<prime<<endl;
	}
	return 0;
}

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

using namespace std;

bool prime_check(int n)
{
	if (n == 1)
	{
		return false;
	}
	else if (n == 2)
	{
		return true;
	}
	else
	{
		for (int i = 2; i <= sqrt(n); i++)
		{
			if (n % i == 0)
			{
				return false;
			}
		}
	}

	return true;
}

int main()
{
	// --- variable define --- //

	int n, c;


	// --- prime check ( 2 to 999 ) --- //

	bool isprime[1000];

	for (int i = 1; i < 1000; i++)
	{
		if (prime_check(i) == true)
		{
			isprime[i] = true;
		}
		else
		{
			isprime[i] = false;
		}
	}


	// --- main --- //

	while (cin >> n)
	{
		c = 0;

		for (int i = 2; i <= n; i++)
		{
			if (i <= 999)
			{
				if (isprime[i] = true)
				{
					c += 1;
				}
				else
				{
					c += 0;
				}
			}
			else
			{
				for (int j = 2; j <= sqrt(i); j++)
				{
					if (n % i == 0)
					{
						c += 0; goto Exit;
					}
				}

				c += 1;
			}
		Exit:;
		}

		cout << c << endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;

#define REP(i, s, n) for (int i = s; i < n; ++i)
#define rep(i, n) REP(i, 0, n)
#define SORT(c) sort((c).begin(), (c).end())
#define IINF INT_MAX
#define LLINF LLONG_MAX
#define DEBUG false

int main()
{
    vector<int> prime;
    prime.push_back(2);
    for (long long int i = 3; i < 999999; i++)
    {
        double temp = sqrt(i);
        for (int j = 0;; j++)
        {
            if (temp < prime[j])
            {
                prime.push_back(i);
                break;
            }
            if (i % prime[j] == 0)
            {
                break; //this is not prime
            }
        }
    }
    int n;
    int i;
    while (cin >> n)
    {
        for (i = 0; prime[i] <= n; i++);
        cout << i << endl;
    }
    return 0;
}

C++: #include<iostream>
int main(){
	for(int m,n,p;!(std::cin>>n).eof();){
		if(n<3)
			p=!(n-2);
		else for(m=3,p=1;m<=n;m+=2){
			p++;
			if(m<2)
				p--;
			else{
				for(int i=3;i*i<=m;i+=2)
					if(m%i==0){p--;break;}
			}
		}
		printf("%d\n",p);
	}
}

C++: #include <iostream>
#include <math.h>
using namespace std;
int isPrime(int n){
    int i;
    if(n == 2)  return true;
    else if(n < 2 || n % 2 == 0)    return false;
    else{
        i = 3;
        while(i <= sqrt(n)){
            if(n % i == 0)  return false;
            i += 2;
        }
    }
    return true;
}
 
int main(void){
    int n, i, cnt;
    while(cin >> n){
        cnt = 0;
        for(i = 1; i <= n; i++){
            if(isPrime(i))  cnt++;
        }
        cout << cnt << endl;
    }
    return 0;
}