Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

C++: #include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <cmath>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <iostream>
#include <random>
#include <map>
#include <iomanip>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cassert>
#include <fstream>
#include <unordered_map>
#include <cstdlib>
#include <complex>
#include <cctype>
#include <bitset>
using namespace std;

using ll = long long;
using vll = vector<long long>;
using pll = pair<long long, long long>;
#define rep(i,n) for(long long i(0);(i)<(n);(i)++)
ll INF = 1LL << 60;

//* calculate primes using sieve
ll sieve(ll n, vll &prime, vector<bool> &isprime){
    //* return the number of primes <= n
    //* prime[0]~ prime[p-1] record the primes
    //* isprime[0] ~ isprime[n] records the sieve
    ll p = 0;
    rep(i, n+1)
        isprime[i] = true;
    isprime[0] = isprime[1] = false;
    for(ll i=2; i <= n; i++){
        if(isprime[i]){
            prime[p++] = i; 
            for(ll j= 2*i; j <= n; j+= i)
                isprime[j] = false;
        }
    }
    return p;
}
int main(){
    vll prime(1000000);
    vector<bool> isprime(1000000);
    ll num = sieve(999999LL, prime, isprime);
    vll rec(1000000, 0);
    for(ll i=2; i< 1000000; i++){
        if(isprime[i])
            rec[i] = 1;
    }
    for(ll i=1; i< 1000000; i++){
        rec[i]+= rec[i-1];
    }

    ll n;
    while(cin >> n){
        cout << rec[n] << endl;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
void multiple(int p,int* a,int n){
	for(int i=2*p;i<=n;i+=p){
		a[i]=1;
	}
}
int main() {
	int n;
	while(cin>>n){
		int a[n+1]={0};
		int count=0;
		for(int i=2;i<=n;i++){
        	if(a[i]==0){
        		count++;
        		multiple(i,a,n);	
        	}  		
		}
		cout<<count<<endl;
	}
	return 0;
}

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

int main(){

	int n, i, j, prmnum;
	bool prm = true;

	while(cin>>n){
		if(n==0)prmnum=0;
		else if(n==1)prmnum=0;

		else{

			prmnum = 1;

			for( i=3;i<=n;i+=2 ){
				prm = true;

				for( j=3;j<=sqrt((double)i);j+=2 ){
					if( i%j == 0 ){
						prm = false;
						break;
					}
				}


				if(prm)prmnum++;
			}
		}


		cout << prmnum << endl;
	}
	return 0;
}

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;
}