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 <cstdio>
#define max_n 999999
int prime[max_n];
bool is_prime[max_n+1];
int main(){
    for(int i=0; i<max_n; i++){
        is_prime[i]=true;
    }
    int p = 0;
    is_prime[1]=false;
    is_prime[0]=false;
    for(int i=2;i<max_n;i++){
        if(is_prime[i]){
            prime[p++] = i;
            for(int j=2*i;j<max_n;j+=i)is_prime[j]=false;
        }
    }
    int n;
    while(scanf("%d",&n)!=EOF){
        printf("%d\n",n);
        int ans = 0;
        for(int i =0; i<max_n; i++){
            ans = i;
            if(prime[i]>n){
                break;
            }
        }
        printf("%d\n",ans);
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>

int count_prime( unsigned int const& n )
{
    if ( n <= 0 || 1000000 <= n ) return -1;
    std::vector<bool> memo_prime( n+1, true );
    for ( unsigned int i = 2; i < n; ++i )
    {
        for ( unsigned int j = 2; i*j <= n; ++j )
        {
            memo_prime.at(i*j) = false;
        }
    }
    memo_prime.at(0) = memo_prime.at(1) = false;
    memo_prime.at(2) = true;
    return std::count( memo_prime.begin(), memo_prime.end(), true );
}

int main()
{
    int n;
    while( std::cin >> n )
        std::cout << count_prime(n) << std::endl;
    return 0;
}

C++: #include<iostream>
using namespace std;
#define N 200000
int main()
{
    int a[N]={0},n,i,j,z;
    for(i=2;i<=N/2;i++){
	if(a[i]==0){
	    for(j=2;j*i<=N;j++){
		a[i*j]=1;	     
    	    }
	}
    }
    while((cin>>n)!=0){
    	z=0;
	for(i=2;i<=n;i++){
    	    if(a[i]==0)z++;
	}
	cout<<z<<endl;
    }
    return 0;	
}

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

int main(){
  int n,i,j;
  while(cin >> n) {
    if(n==1){
      cout << 0 <<endl;
      continue;
    }
    int k = 0, a[n];
    a[0] = 2;
  
    for (i = 3; i <= n; i = i + 2) {
      for (j = 0; j <= k; j++) {
        if(i % a[j] == 0 ) break;
        if(i <= a[j] * a[j]) {
          k++;
          a[k] =i;
          break;
        }
        
      }
      if(j == k + 1) {
        k++;
        a[k] = i;
      }
    }
    cout << k+1 << endl;
  }
  
  return 0;
}

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

int whether(int num);
int count(int num);

int main(){
	int n=1;
	int a=1;

	while(a==n){
	scanf("%d",&n);
	a = count(n);
	printf("%d\n",a);
	a=n;
}

scanf("%d",&a);
	return 0;
}

int whether(int num){
	int s=sqrt(num);
	int i;
	for(i=3; i<=s; i+=2){ if(num%i==0) return 0; }

	return 1;
}

int count(int num){
	int a=1;
	int i;
	if(num <2){ return 0; }
	for(i=3; i<=num; i+=2){
		a += whether(i);
	}

	return a;
}

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

#define rep(i,n) for(int i=2; i<=n; i++)
#define rep2(i,n) for(int i=0; i<n; i++)
bool isprime(int n){
	rep(i,sqrt(n)){
		if(n%i==0) return false;
	}
	return true;
}
int main(){
	int n, ans;
	bool table[1000000];
	rep2(i,1000000) table[i]=true;
	rep(i,1000000){
		if(table[i] && isprime(i)){
			rep(j,999999/i) table[i*j]=false;
		}
	}
	while(cin >> n){
		ans=0;
		rep(i,n) if(table[i]) ans++;
		cout << ans << endl;
	}
    return 0;
}

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

int main()
{
	unsigned int  n,j;
	cin >> n;
	unsigned  int num[n],count = 0;
	for(int i = 0; i < n; i++)
	{
		cin >> num[i];
	}
	for(int i = 0; i < n; i++)
	{
		for(j = 2; j <num[i]; j++)
		{
			if((num[i] % j) == 0)
			{
				break;
			}
		}
		if(j == num[i]) count++;
  }
	cout << count << endl;
	return 0;
}

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

int main() {
int b[1000000];
for (int k=0;k<1000001;k++){
	b[k]=0;
}
b[2]=1;
b[3]=1;
b[4]=0;
b[5]=1;
b[6]=0;
b[7]=1;

for (int p=11;p<1000001;p+=2){
int i,a=0 ;
	i=3;
	while(i<sqrt(p)){
		if(p%i==0){
			break;
	}i=i+2;
	}
	if (i>=sqrt(p)){
		a=1;
	}
	b[p]=a;
}
int c[1000000];
for (int l=0;l<1000001;l++){
	c[l+1]=c[l]+b[l+1];
}
int r;
while(cin>>r){
	cout<<c[r]<<endl;
}
	return 0;
}

C++: #include <iostream>

using namespace std;

#define MAX 1000000

int main(){
  int n;
  bool p[MAX];
  p[0] = false;
  p[1] = true;
  for(int i=2;i<MAX;i++)p[i] = true;
  for(int i=2; i*i<MAX;i++){
	if(p[i]){
	  for(int j=i*i;j<=MAX;j=j+i){
		p[j] = false;
	  }
	}
  }
  
  while(cin >> n){
	int cnt = 0;
	for(int i = 0;i<n;i++){
	  if(p[i]) cnt++;
	}
	cout << cnt << endl;
  }
  
  return 0;
}

C++: #include<iostream>

using namespace std;

int n;

bool PrimeJudge(int n)
{
	bool judge = true;
	for(int i=2;i<n;++i)
	{
		if(n%i>1)
		{
			judge = false;
			break;
		}
	}
	return judge;
}

int main()
{
	while(cin>>n)
	{
		int count=0;
		for(int i=1;i<=n;++i)
		{
			if(PrimeJudge(i))
			{
				++count;
			}
		}
		cout<<count<<endl;
	}
}

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

using namespace std; 

int list[1000000]={2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73,
79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163,
 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251,
 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349,
 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443,
 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557,
 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647,
 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757,
 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863,
 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983,
 991, 997, 1009, 1013, 1019, 1021, 1031, 1033, 1039, 1049, 1051, 1061, 1063, 1069};

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

 bool Prima(unsigned long n) 
{
    for (unsigned short i = 0; list[i]*list[i] <= n; i++) 
	{
        if (n % list[i] == 0) 
		{
            return false;
        }
    }
 
    return true;
}

int main()
{
	lo=1087;
	a=180;
	while(lo<=100003)
	{
		if(Prima(lo))
		{
			list[a]=lo;
			a++;
		}
		lo+=2;
	}
	//printf("%d",list[78498]);
	while(scanf("%ld",&input)!= EOF)
	{	
	lo=0;
	hi=78497;
	/*while(lo<=hi)
        {
            mid=(lo+hi)/2;
            if (list[mid]<input)
            {
                lo=mid+1; 
            } 
            else if(list[mid]>input)
            {
                hi=mid-1; 
            }
            else 
            {
                break; 
            } 
        }*/
	while(input>=list[lo])
	{
		lo++;
	}
	printf("%d\n",lo);
	}
	return 0; 
}