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++: #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;
}

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

int main(){
  int a;
  int i, j, k;
  int past=1, a_sqrt;
  int count;
  bool chk[1000000];

  for(i=1; i<1000000; i++) chk[i]=true;
  while(cin>>a){
    count=0;
    if(past>=a){
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
    else {
      a_sqrt=(int)sqrt((double)a);
      for(i=2; i<=a_sqrt; i++){
        k=past/i; k++;
        for(j=k; j<=a/i; j++){
          chk[j*i]=false;
        }
      }
      past=a;
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
  }

  return 0;
}

C++: #include <assert.h>
#include <ctype.h>
#include <float.h>
#include <limits.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

#include <algorithm>
#include <complex>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
using namespace std;

#define SZ(a) (int)(a).size()
#define FOR(i,a,b) for (int i=(a); i<=(b); ++i)
#define REP(i,n) for (int i=0; i<(n); ++i)
#define ALL(c) c.begin(), c.end()
#define CLR(c,n) memset(c, n, sizeof(c))
#define TR(it, c) for(typeof(c.begin()) it = c.begin();it != c.end(); ++it)
#define CONTAIN(it, c) (c.find(it) != c.end())
typedef vector<int> VI;
typedef pair<int, int> PII;
template <class T> void checkmin(T &a, T b) { if (b<a) a=b; }
template <class T> void checkmax(T &a, T b) { if (b>a) a=b; }
const int INF=0x3F3F3F3F;

const int N=1<<20;
int p[N];
int main(int argc, char *argv[]) {
  p[0] = p[1] = 0;
  fill(p + 2, p+N, 1);
  for (int i = 2; i * i < N; ++i) {
    if (p[i]) {
      for (int j = i * i; j < N; j += i) {
        p[j] = 0;
      }
    }
  }
  FOR(i, 3, N-1) p[i] += p[i-1];
  int n;
  while (scanf("%d", &n) == 1) printf("%d\n", p[n]);
}

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

int main(){
	int n,s,a;
	
	while(cin >> n){
		a=1;
		for(int j=3;j<=n;j+=2){
			s=0;
			for(int i=3;i<j;i+=2){
				if(j%i==0){
					s++;
				}
			}
			if(s==0){
				a++;
			}
		}
		cout << a << "\n";
	}
		
	
return 0;
}

C++: #include <iostream>
using namespace std;
bool PrimeCheck(int n) {
  int i = 2;
  while(i < n) {
	if(n % i == 0) return false;
	i++;
  }
  return true;
}
int main() {
  int n;
  while(cin >> n) {
	int cnt = 0;
	for(int j = 2; j <= n; ++j) {
	  if(PrimeCheck(j)) cnt++;
	}
	cout << cnt << endl;
  }
  return 0;
}

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

bool is(float n)
{
	for(int i=2;i<=sqrtf(n);i++)
		if((int)n%i==0)
			return false;
	return true;
}

int main()
{
	int n;
	while(cin>>n)
	{
		int sum=1;
		for(int i=3;i<=n;i++)
			if(is(i)==true)
				sum++;
		cout<<sum<<endl;
	}
}