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>
using namespace std;

int main(){
	int n;
	int sosu[10000];
	int snum;
	bool judge;

	sosu[0] = 2;

	while(cin >> n){
		snum = 0;
		for(int i = 3; i <= n; i++){
			judge = true;
			for(int j = 0; j <= snum; j++){
				if(i%sosu[j] == 0){
					judge = false;
					break;
				}
			}
			if(judge == true){
				snum ++;
				sosu[snum] = i;
			}
		}
		cout << snum + 1 << endl;
	}



	return(0);
}

C++: #include<cstdio>
#include<cstring>

const int N = 1000009;
int dp[N];
int isprime[N];

int main()
{
    dp[1] = 0;
    for(int i=2;i<N;i++)
    {
        if(!isprime[i])
            for(int j=2*i;j<N;j+=i)
                isprime[j] = 1;
        dp[i] = dp[i-1] + ((isprime[i])?0:1);
    }

    int y;
    while(~scanf("%d",&y))
        printf("%d\n",dp[y]);
    return 0;
}

C++: #include "bits/stdc++.h"

#define REP(i,num) for(int i=0;i<(num);++i)
#define ALL(c) c.begin(),c.end()
#define PRINTALL(c) for(auto& x:c){cout<<x<<' ';}cout<<endl;
#define PAIRCOMP(c,comp) [](const pair<ll,ll>& lhs,const pair<ll,ll>& rhs){return lhs.c comp rhs.c;}

using namespace std;
using ll = long long;

constexpr ll atcoder_mod = 1e9+7;

template<typename T=int>
T in(){T x; cin >> x; return (x);}
template<typename T=int,typename C=vector<T>>
C vecin(int N){C x(N);REP(i,N){x[i]=in<T>();}return move(x);}

void vout(){cout << endl;}
template<typename Head,typename... Tail>
void vout(Head&& h,Tail&&... t){cout << ' ' << h;vout(forward<Tail>(t)...);}
void out(){cout << endl;}
template<typename Head,typename... Tail>
void out(Head&& h,Tail&&... t){cout << h;vout(forward<Tail>(t)...);}

vector<ll> enum_prime(ll N){
	deque<bool> is_prime(N+1,true);
	vector<ll> P;
	for(ll i=2;i<=N;i++){
		if(is_prime[i]){
			for(ll j=i*i;j<=N;j+=i){
				is_prime[j] = false;
			}
			P.emplace_back(i);
		}
	}
	return P;
}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int N;
	while(cin>>N){
		auto P=enum_prime(N);
		out(P.size());
	}

	return 0;
}

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

C++: #include<stdio.h>

int main()
{
    int i,j,n,m,a[10000000],c=0,k,l=0;
    while (scanf ("%d", &n) != EOF) 
    {
    a[c]=n;
    c++;
    }
    for(k=0;k<c;k++)
    {
    l=0;
        for(j=1;j<=a[k];j++)
        {
            m=j;
            for(i=2;i<m;i++)
            {
                if(m%i==0)
                    break;
            }
            if(m==i)
                l++;
        }
         printf("%d\n",l);
    }
    return 0;
}

C++: #include <iostream>

using namespace std;
int main(void){
    int n;
    int count;
    int prime[1000000];
    int i,j;
    while(cin>>n){
        count=0;
        fill(prime,prime+1000000,0);
        prime[0]=prime[1]=1;
        for(i=2;i<=n;i++){
            if(prime[i]==0){
                count++;
                for(j=2*i;j<=n;j+=i){
                    prime[j]=1;
                }
            }
        }
        cout<<count<<endl;
    }
    return(0);
}

C++: #include<iostream>
 
using namespace std;
 
int main(){
  static bool cmp[1000000]={true};
  static int npr[1000000];
  for(int i=2;i<1000000;i++){
    if(!cmp[i]){
      for(int j=2;j*i<1000000;j++){
    cmp[i*j]=true;
      }
    }
    npr[i]=npr[i-1]+!cmp[i];
  }
  for(int n;cin>>n;n++){
    cout<<npr[n]<<endl;
  }
  return 0;
}

C++: #include <iostream>
#include <vector>
#define MAX 1000000
using namespace std;

int main(){
  //bool num[MAX];
  pair<bool,int> num[MAX];
  int input=0;
  while(cin >> input){
   int count=0;
  for(int i=2; i<MAX; i++){
    num[i].first=true;
    num[i].second=i;
  }
  for(int i=2;i<input+1;i++){
    if(num[i].first){
      //cout << num[i].second <<",";
      count++;
      for(int j=i+i; j<MAX; j+=i){
        num[j].first=false;
      }
    }
  }
  cout << count << endl;
  }
}

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

bool isPrime(int num) {
	if (num == 1)return false;
	if (num == 2)return true;


	if (num % 2 == 0)return false;
	for (int i = 3; i *i <= num; i += 2) {
		if (num % i == 0)return false;
	}
	return true;
}


int main() {

	int num;
	int count;
	while (cin >> num) {
		count = 0;
		if (num >= 2)count++;
		for (int i = 1; i <= num; i += 2) {
			if (isPrime(i))count++;
		}
		printf("%d\n", count);

	}

	//system("pause");
	return 0;
}

C++: #include <iostream>
#include <bitset>
#include <vector>
#include <cmath>
#define	MAXPRIME	999999
using namespace std;

typedef unsigned int uint;

void setPrimeFlag(bool *primeFlag, uint maxPrime) {
	uint sq = (uint)sqrt((double)maxPrime) + 1;
	for (uint i = 2; i <= maxPrime; ++i) { primeFlag[i] = true; }
	primeFlag[0] = primeFlag[1] = false;
	for (uint i = 2; i <= sq; ++i) {
		if (primeFlag[i]) {
			for (uint j = i + i; j <= maxPrime; j += i) {
				primeFlag[j] = false;
			}
		}
	}
}

void setPrimeArray(vector<uint> &primeArray, uint maxPrime) {
	bool *primeFlag = new bool[maxPrime + 1];
	setPrimeFlag(primeFlag, maxPrime);

	primeArray.clear();
	for (uint i = 2; i <= maxPrime; ++i) {
		if (primeFlag[i]) {
			primeArray.push_back(i);
		}
	}
	delete [] primeFlag;
}

int main() {
	vector<uint> primeArray;
	setPrimeArray(primeArray, MAXPRIME);

	for (;;) {
		uint n, num;
		cin >> n;
		if (cin.eof()) { break; }

		for (num = 0; num < primeArray.size() && primeArray[num] <= n; ++num);

		cout << num << endl;
	}

	return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;

int pn[1000002];

int main(){
int n, Count;
cin.tie(0);
while(scanf("%d",&n)!=EOF){
Count=0;
memset(pn,0,n);
pn[0]=pn[1]=1;
for(int i=2;i<=n;i++) if(!pn[i]) for(int j=1;i*j<=n;j++) pn[i*j]=1;
Count=count(pn,pn+n,0);
cout<<Count<<endl;
}
return 0;
}

C++: #include<iostream>

using namespace std;

int main(){
    int n;
    while (cin >> n){
        int result = 0;
        for (int i = 2; i <= n; i++){
            for (int j = 2; j <= i; j++)
            {
                if (i == j)result++;
                if (i%j == 0)break;
            }
        }
        cout << result << endl;
    }


    return 0;
}

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

int main() {
	int n;
	
	while (cin >> n) {
		bool prime[1000000];
		for (int i = 2;i <= n;i++) {
			prime[i] = true;
		}
		for (int i = 2;i <= sqrt(n);i++) {
			for (int j = i * 2;j <= n;j += i) {
				prime[j] = false;
			}
		}
		int count = 0;
		for (int i = 2;i <= n;i++) {
			if (prime[i] == true) count++;
		}
		cout << count << endl;
	}

}

C++: /// http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0009
/// http://www.tsg.ne.jp/TT/tsg/c/mansaku/P01.html
#include<cstdio>
#include<cmath>

int main(){
    int n, cnt=0;
    while(~scanf("%d", &n)){
        for(int i = 3; i<=n; i+=2){
            int k= 0;
            for(int j=3; j<sqrt(i); j+=2){
                if(i%j==0){
                    k=1;
                    break;
                }
            }
            if(k==0) cnt++;
        }
        printf("%d\n", cnt);

    }

    return 0;
}