Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

func next()string{
	return sc.Text()
}

func nextInt()int{
	a, _ := strconv.Atoi(next())
	return a
}

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

int prime(int n) {
    if (n == 2) return 1;
    if (n % 2 == 0) return 0;
    for (int i = 3; i <= n / i; i += 2) {
        if (n % i == 0) return 0;
    }
    return 1;
}

int main() {

    int C[1000000] = {0};
    for (int i = 2; i < 1000000; ++i) {
        C[i] = prime(i);
    }
    for (int i = 2; i < 1000000; ++i) {
        C[i] += C[i - 1];
    }

    int n;
    while (cin >> n) {
        cout << C[n] << endl;
    }

    return 0;
}

C++: #include<iostream>
using namespace  std;
bool isPrime(int p)
{
	bool flag = true;
	for (int i = 2; i * i <= p; i++)
	{
		if (p % i == 0)
		{
			flag = false;
			break;
		}
	}
	return flag;
}
int main()
{
	int n;
	while (cin >> n)
	{
		int count = 0;
		for (int i = 2; i <= n; i++)
		{
			if (isPrime(i))count++;
		}
		cout << count << endl;
	}
	return 0;
}

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

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

C++: #define _USE_MATH_DEFINES

#include <iostream>
#include <string>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <algorithm>
#include <utility>
#include <vector>
#include <numeric>

using namespace std;

bool is_prime(int n)
{
	if (n == 2)
	{
		return true;
	}
	else if ((n == 1) || (n % 2 == 0))
	{
		return false;
	}

	int p = (int)sqrt(n);
	for (int i = 3; i <= p; i += 2)
	{
		if (n % i == 0)
		{
			return false;
		}
	}

	return true;
}

int main()
{
	vector<int> p;
	for (int i = 1; i < 1000000; i++)
	{
		if (is_prime(i))
		{
			p.push_back(i);
		}
	}

	int n;
	for (; cin >> n;)
	{
		int i;
		for (i = 0; i < p.size(); i++)
		{
			if (p.at(i) > n)
			{
				break;
			}
		}

		cout << i << endl;
	}
}

C++: #include <math.h>
#include <iostream>
#include <ios>
#include <iomanip>
#include <algorithm>
#include <functional>
#include <string>

#define YES "YES"
#define NO "NO"
#define space ' '
#define not true
#define prime false

using namespace std;

void init();
void func();

int main(void)
{
	
	func();

	return 0;
}


#define MAXIMUM 1000000
bool table[MAXIMUM];
int cnt[MAXIMUM];
#define not true
#define prime false

void init()
{
		table[0] = not;
	table[1] = not;
	for (int i = 2; i < MAXIMUM; i++)
	{
		if (table[i] == prime)
		{
			for (int j = i * 2; j < MAXIMUM; j += i)
			{
				table[j] = not;
			}
		}
	}
	for (int i = 1; i < MAXIMUM; i++)
	{
		if (table[i] == prime)
		{
			cnt[i] = cnt[i - 1] + 1;
		}
		else
		{
			cnt[i] = cnt[i - 1];
		}
	}

}

void func()
{
	
	int n;

	init();

	while (cin >> n, !cin.eof())
	{
		cout << cnt[n] << endl;
	}

}

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

int main(){
  int n,m,p[999999];
  bool ip[1000000];
  for(int i=0;i<=1000000;i++) ip[i]=true;
  ip[0]=ip[1]=false;
  for(int i=2;i<=1000000;i++){
    if(ip[i]){
      p[m++]=i;
      for(int j=2*i;j<=1000000;j+=i) ip[j]=false;
    }
  }
  while(cin>>n){
    for(int i=0;i<=m;i++){
      if(p[i]>n){
	cout<<i<<endl;
	break;
      }
    }
  }
  return 0;
}

C++: #include<stdio.h>

int main(void)
{
	int data,i=0,j=2,k=0,l=0,kai[100]={0},s[10000]={0};

	while(scanf("%d",&data) != EOF)
	{
		if(data<=1)
		{
			kai[k]=0;
		}
		else
		{
			for(i=2;i<data;i++)
				s[i]=i;
		
			for(i=2;i*2<data;i++)		
			{
				for(j=2;j*i<data;j++)				
					s[j*i]=0;
			}
			for(i=2;i<data;i++)
			{	
				if(s[i]!=0)
				{
					l+=1;
				}
			}
		}
		kai[k]=l;
		k+=1;
	}

  for(j=0;j<k;j++)
    printf("%d\n",kai[j]);
  return 0;
}

C++: #include <stdio.h>
#include <string>
#include <iostream>

//using namespace std;

int sosuu(int n);

int main(){
    int n = 0;
    int temp = 0;
    while(scanf("%d",&n)==EOF){
        if(n==1){
            printf("%d",0);
        }else{
            for(int i=2; i<=n; i++ ){
                temp+=sosuu(i);
            }
            printf("%d",temp);
        }
    }
    
    
}

int sosuu(int n){
    for(int i=2; i<n; i++){
        if(n%i == 0){
            return 0;
        }
    }
    return 1;
}

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

using namespace std; 

long int a,b,loop,hasil;
long int lo;
long int hi;



int main()
{
	while(scanf("%ld",&hi)!= EOF)
	{
	lo=2;
	hasil=0;
		while(lo<=hi)
		{
			for(a=2;a<=lo;a++)
			{
				if(a==lo)
				{
					hasil++;
					break;
				}
				else if(lo%a==0)
				{
					break; 
				}
				else 
				{
				}
			}
			lo=lo+1; 
		}
	printf("%d\n",hasil);
	}
	return 0; 
}

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

int main(){

	const int N_MAX=1000000;
	int i,j,n[32],ni,max;
	int count[N_MAX];
	bool so[N_MAX];
	
	for(i=0;i<N_MAX;i++)
		so[i]=true;
	ni=0;
	max=0;
	while(cin>>n[ni])
		if(n[ni++]>max)
			max=n[ni-1];
	so[0]=so[1]=0;
	for(i=2;i<=1000;i++){
		if(so[i]==0)continue;
		for(j=i*2;j<=max;j+=i)
			so[j]=false;
	}
	for(i=2;i<=max;i++){
		count[i] = count[i-1]+so[i];
	}
	for(i=0;i<ni;i++)
		cout << count[n[i]] << endl;

	return 0;
}