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<bits/stdc++.h>
using namespace std;
 
#define maxPN 1000000
#define PNs  1000100
int countPN = 0, countPNs = 0;
long long PrimeNum[maxPN], temp;
bool PN[PNs];
 
bool IsPrimeNumber(long long x);
 
void PrimeN(){
    int i, j;
    PrimeNum[0] = 2;
    PN[2] = true;PrimeNum[countPN++] = 2;
    PN[3] = true;PrimeNum[countPN++] = 3;
//  PN[5] = true;PrimeNum[countPN++] = 5;
     
    for(i = 6; i  < PNs;i+=6){
        if(IsPrimeNumber(i-1)){
            PrimeNum[countPN++] = i-1;
            PN[i-1] = true;//cout<<i-1<<endl;
        }
        if(IsPrimeNumber(i+1)){
            PrimeNum[countPN++] = i+1;
            PN[i+1] = true;//cout<<i+1<<endl;
        }
        countPNs = i+1;//cout<<i-1<<" "<<PN[i-1]<<endl;
    }
}
 
bool IsPrimeNumber(long long x){
    //if(x == 2 || x == 3) return true;
    //if(x <= 4 || x%2 == 0 || x%3 == 0)   return false;
    //if(x < countPNs) return PN[x];
    for(long long i = 0; PrimeNum[i] * PrimeNum[i] <= x; i++)
        if(x%PrimeNum[i] == 0)return false;
    return true;
}
 
 
int main(){
    clock_t start, end;
    long long n = true, counta;
    start = clock();
    PrimeN();
//  cout<<PrimeNum[countPN-1]<<" "<<countPNs<<endl;
//  cout<<clock()-start<<"ms"<<endl;
     
    while(cin>>n){
        counta = 1;
         
         
        //cout<<(IsPrimeNumber(n)?"yes":"no")<<endl;
        start = clock();
        //cout<<IsPrimeNumber(n)<<endl;
        for(int i = 1; i <= n; i+=2){
            if(PN[i])counta++;
        //  cout<<i<<" "<<PN[i]<<endl;
        }
        if(n == 1) counta = 0;
        cout<<counta<<endl;
    //  cout<<clock()-start<<"ms"<<endl;
    }
    return 0;

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

#define rep(i,x,y) for(int i=(x);i<(y);++i)
#define debug(x) #x << "=" << (x)

#ifdef DEBUG
#define _GLIBCXX_DEBUG
#define show(x) std::cerr << debug(x) << " (L:" << __LINE__ << ")" << std::endl
#else
#define show(x)
#endif

typedef long long int ll;
typedef pair<int,int> pii;
template<typename T> using vec=std::vector<T>;

const int inf=1<<30;
const long long int infll=1LL<<62;
const double eps=1e-9;
const int dx[]={1,0,-1,0},dy[]={0,1,0,-1};

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec){
    os << "[";
    for (const auto &v : vec) {
    	os << v << ",";
    }
    os << "]";
    return os;
}

void init_sieve(vector<bool> &is_prime,int n){
	for(int i=0; i<n; ++i) is_prime[i]=true;
	is_prime[0]=is_prime[1]=false;
	for(int i=2; i*i<=n; ++i){
		if(is_prime[i]){
			for(int j=2*i; j<n; j+=i) is_prime[j]=false;
		}
	}
}

vector<bool> is_prime(1e6,true);

void solve(int n){
    int ans=0;
    rep(i,0,n+1) if(is_prime[i]) ++ans;
    cout << ans << endl;
}

int main(){
    std::cin.tie(0);
    std::ios::sync_with_stdio(false);
    cout.setf(ios::fixed);
    cout.precision(10);
    init_sieve(is_prime,1e6);
    int n;
    while(cin >> n) solve(n);
    return 0;
}