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<iostream>
#include<cstring>
#include<cmath>
using namespace std;

int main(void){
  int n;
  bool p[1000000];
  p[0] = p[1] = 0;
  p[2] = 1;
  int ans = 0;
  bool flag;
  for(int i = 0; i < 1000000; i++)
    p[i] = false;
  p[2] = true;
  for(int i = 3; i <= 1000000; i += 2){
    flag = true;
    for(int j = 3; j * j <= i; j += 2){
      if(i % j == 0){
	flag = false;
	break;
      }
    }
    if(flag)
      p[i] = true;
  }
  // cout << "end" << endl;
  while(cin >> n){
    int ans = 0;
    for(int i = 2; i < n; i++){
      if(p[i])
	ans++;
    }
    cout << ans << endl;
  }
  return 0;
}

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

int main(){
    int maxy = (int)sqrt(100000);
    int prime[100000];
    for(int i = 0; i < 100000; i++){
        prime[i] = 1;
    }
    prime[0] = prime[1] = 0;
    for(int i = 2; i <= maxy; i++){
        if(prime[i] == 1){
            for(int j=i*2; j <= maxy; j+=i){
                prime[j] = 0;
            }        
        }
    }
    int n;
    while(cin >> n){
        int ans = 0;
        for(int i=0; i <= n; i++){
            if(prime[i] == 1){
                ans++;
            }
        }
        cout << ans << endl;
    }
    return 0;
}

C++: #include<iostream>

using namespace std;

int main(){
  int n,flag=1,sum = 0;

  while(cin >> n){

    for(int i=2; i <= n; i++){
      if(i != 2 && i%2 == 0) flag = 0;
      else if(i != 3 && i%3 == 0) flag = 0;
      else if(i != 5 && i%5 == 0) flag = 0;
      else{
	for(int j=3; j < i/2; j+2){
	  if(i%j == 0 ) flag = 0;
	}
      }
      if(flag == 1) sum++;
      flag = 1;
    }
    cout << sum <<endl;
    sum = 0;
  }
}

C++: #include <stdio.h>

const int MAX_V = 100000;
int prime[MAX_V+1];

void init(){
    int i, k, v;
    for(i=2; i<MAX_V; i++){
        prime[i] = 1;
    }
    for(i=2; i*i<=MAX_V; i++){
        if(prime[i]){
            for(k=2*i; k<=MAX_V; k+=i){
                prime[k]=0;
            }
        }
    }   
}

int isprime(int n){
    return prime[n];
}

int main(){
    init();
    int num;
    while(scanf("%d", &num) != EOF){
        int count=0;
        for(int i=2; i<=num; i++){
            if(isprime(i)==1)
                count++;
        }
    printf("%d\n", count);
    }
    return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;

#define LOG(...) fprintf(stderr, __VA_ARGS__)
//#define LOG(...)
#define FOR(i, a, b) for(int i=(int)(a); i<(int)(b); ++i)
#define REP(i, n) for(int i=0; i<(int)(n); ++i)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define EXIST(s, e) ((s).find(e)!=(s).end())
#define SORT(c) sort(ALL(c))
#define RSORT(c) sort(RALL(c))
#define SQ(n) (n) * (n)

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int MAX_VALUE = 100007;
bool prime[MAX_VALUE];

int main() {
  FOR(i, 2, MAX_VALUE){
    if(prime[i]) continue;
    for(int j = i * 2; j<MAX_VALUE; j+=i)
      prime[j]=true;
  }

  int n;
  while(cin >> n) {
    int cnt = 0;
    FOR(i, 2, n + 1){
      if (!prime[i]) cnt++;
    }
    cout << cnt << endl;
  }
}

C++: #include <algorithm>
#include <bitset>
#include <cassert>
#include <complex>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <typeinfo>
#include <utility>
#include <vector>
#include <array>
#include <chrono>
#include <random>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;Scan(__VA_ARGS__);
template<typename T> void Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void Scan(First& first, Rest&...rest) { std::cin >> first; Scan(rest...); }
#define OUT(d) std::cout<<d;
#define FOUT(n, d) std::cout<<std::fixed<<std::setprecision(n)<<d;
#define SOUT(n, c, d) std::cout<<std::setw(n)<<std::setfill(c)<<d;
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& r:c)for(auto& i:r)std::cin>>i;
#define ALL(a) (a).begin(),(a).end()
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-1;i>=(a);--i)
#define REP(i, n) for(int i=0;i<int(n);++i)
#define RREP(i, n) for(int i=(n)-1;i>=0;--i)
#define PAIR std::pair<int, int>
#define IN(a, x, b) (a<=x && x<b)
#define SHOW(d) {std::cout << #d << "\t:" << d << "\n";}
#define SHOWVECTOR(v) {std::cout << #v << "\t:";for(const auto& i : v){std::cout << i << " ";}std::cout << "\n";}
#define SHOWVECTOR2(v) {std::cout << #v << "\t:\n";for(const auto& i : v){for(const auto& j : i){std::cout << j << " ";}std::cout << "\n";}}
#define SHOWPAIRVECTOR2(v) {std::cout << #v << "\t:\n";for(const auto& i : v){for(const auto& j : i){std::cout<<'('<<j.first<<", "<<j.second<<") ";}std::cout << "\n";}}
#define SHOWPAIRVECTOR(v) {for(const auto& i:v){std::cout<<'('<<i.first<<", "<<i.second<<") ";}std::cout<<"\n";}
#define CHECKTIME(state) {auto start=std::chrono::system_clock::now();state();auto end=std::chrono::system_clock::now();auto res=std::chrono::duration_cast<std::chrono::nanoseconds>(end-start).count();std::cerr<<"[Time:"<<res<<"ns  ("<<res/(1.0e9)<<"s)]\n";}
#define SHOWQUEUE(a) {std::queue<decltype(a.front())> tmp(a);std::cout << #a << "\t:";for(int i=0; i<static_cast<int>(a.size()); ++i){std::cout << tmp.front() << "\n";tmp.pop();}std::cout << "\n";}

//#define int ll
using ll = long long;
constexpr int INFINT = 1 << 30;
constexpr ll INFLL = 1LL << 60;
constexpr double EPS = 0.0000000001;
constexpr int MOD = 1000000007;

std::vector<int> getPrimeVec(int n) {
	std::vector<bool> num(n, true);
	std::vector<int> prime;
	num[0] = false;
	for (int i = 0; i<std::sqrt(n); ++i) {
		if (num[i]) {
			for (int j = 2; (i + 1)*j <= n; ++j) {
				num[(i + 1)*j - 1] = false;
			}
		}
	}
	for (int i = 0; i<n; ++i) {
		if (num[i]) {
			prime.push_back(i + 1);
		}
	}
	return prime;
}

int main() {
	INIT;
	std::vector<int> primes(getPrimeVec(1000000));
	int n;
	while (std::cin >> n) {
		OUT(std::distance(std::begin(primes), std::upper_bound(ALL(primes), n)))BR;
	}
	return 0;
}

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

using namespace std;

int main(void)
{
	int n, count,k;

	while(cin >> n){
		count = 1;
		k = 0;

		for(int i = 3; i <= n; i += 2){
			for(int j = 3; j <= sqrt(i/1.0); j += 2)
			{
				if(i%j == 0){
					k = 1;
					break;
				}
			}
			if(k == 0){
				count++;
			}
		}
		cout << count << endl;
	}

	return 0;
}

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

int main(){
	int num, cou, i, j, flag;
	
	while(cin >>num){
		cou = 0;
		for(i = 2; i <= num; i++){
			for(j = 2; j < i; j++){
				if(i % j == 0){
					flag = 1;
					break;
				}
			}
			if(flag == 0) cou++;
			flag = 0;
		}
		if(num % 2 == 0) cou++;
		cout <<cou<<endl;
	}
	
	return 0;
}