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>
#include<math.h>

using namespace std;

int main(){
	int n,i,j,buf,cnt;
	bool flag;

	while(cin >> n){
		if(n == 1){
			cout << "0" << endl;
			continue;
		}

		cnt = 1;

		for(i=3;i<=n;i+=2){
			flag = false;
			buf = (int)sqrt((double)i);

			for(j=3;j<=buf;j+=2){
				if((i % j) == 0){
					flag = true;
					break;
				}
			}

			if(flag == false){
				cnt++;
			}
		}

		cout << cnt << endl;
	}

	return 0;
}

C++: /*#include"iostream"
#include"stdio.h"
#include"string.h"
using namespace std;
long kt( string a)
{
	long d=0, d1=0;
	long n=a.size();
	if(97<=a[0]&&a[0]<=122)
	{
			for(long i=1;i<n;i++)
			if(65<=a[i]&&a[i]<=90)
			d++;
	}
	
	for(long i=0;i<n;i++)
	{
		if(65<=a[i]&&a[i]<=90)
		d1++;
	}
	if(d1==n) return 2;
	if(d==n-1) return 1;
	return 0;
}
long main()
{
	
	string a;
	cin>>a;
	long n=a.size();
	if(kt(a)==2)
	{
		for(long i=0;i<n;i++)
		a[i]+=32;
		cout<<a;
		return 0;
	}
	if(kt(a)==1)
	{
		for(long i=1;i<n;i++)
		a[i]+=32;
		a[0]-=32;
		cout<<a;
		return 0;
	}
	else cout<<a;
}
*/

#include <iostream>
using namespace std;

#define long long long
const long N = 10000000;
bool np[N];
long p[N], pp=0;

void eratos() {
    np[0]=np[1]=true;
    for (long i=2; i*i<N; i++) if (!np[i])
        for (long j=i*i; j<N; j+=i) np[j]=true;
    for (int i=2; i<N; i++)
    if (!np[i]) p[++pp]=i;
}
main()
{
	eratos();
	long n,d=0;
do
{
     d=0;
	cin>>n;
	
	for(long i=1;i<=n;i++)
	if(p[i]>n)
	{cout<<i-1<<endl; 
	break;}
}while(1);
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
#include <stack>
#include <fstream>
#include <vector>
#include <map>
#include <set>
#include <cmath>
#include <algorithm>
#include <functional>
#define inf 0x3f3f3f3f
#define inff -30000000
using namespace std;
typedef long long ll;
const int MAXN=1e5+10;
const int MAX=1000000+10;
bool notprime[MAX];
int num=0;
void init(){
    memset(notprime,0,sizeof(notprime));
    for(int i=2;i*i<=MAX;i++){
        if(!notprime[i]){
            for(int j=i*i;j<=MAX;j+=i)
                notprime[j]=1;
        }
    }
}
int main(){
    #ifdef SIYU
    freopen("in.txt","r",stdin);
    #endif // SIYU
    int n;
    init();
    while(cin>>n){
        //int *p=upper_bound(prime,prime+num,n);
        //int ans=p-prime;
        //cout<<ans<<endl;
        int ans=0;
        for(int i=2;i<=n;i++){
            if(!notprime[i])
                ans++;
        }
        cout<<ans<<endl;
    }
    return 0;
}

C++: #include<stdio.h>
#define N 1000010

int main() {
	int ipt;
	int root;
	int cnt = 0;
	int A[N];
	for (int i = 1; i <= N; i++) {
		A[i] = i;
	}
	for (int pos = 2; pos <= N; pos++) {
		root = A[pos];
		if (!root)continue;
		for (int k = 2; k*root <= N; k++) {
			A[k*root] = 0;
		}
	}

	for (int P = 0; P < 30;P++) {
		scanf("%d", &ipt);
		for (int i = 2; i <= ipt; i++) {
			if (A[i] != 0)cnt++;
		}
		printf("%d\n", cnt);
		cnt = 0;
	}

	return 0;
}

C++: #include<stdio.h>
#include<math.h>
int gurobal[100000001]={0};
int main(void)
{
	int n=0;
	int i=0,j=0;
	int heihoukon=0;
	int count=0;
	while(scanf("%d",&n)!=EOF)
	{
		count=0;
		gurobal[1]=0;
		for (i=2;i<=n;i++)
		{
			gurobal[i]=1;
		}
		(int)heihoukon=sqrt((double)n);
		for (i=2;i<=heihoukon;i++)
		{
			if (gurobal[i]!=0)
			{
				for (j=i*2;j<=n;j+=i)
				{
					gurobal[j]=0;
				}
			}
		}
		for (i=2;i<=n;i++)
		{
			if (gurobal[i]!=0)
			{
				count++;
			}
		}
		printf("%d\n",count);
	}
	return 0;
}

C++: #include<cstdio>
const int maxn = 1000010;
int prime[maxn + 10];
int main(){
	for(int i = 2; i <= maxn; i++){
		if(!prime[i]) prime[++prime[0]] = i;
		for(int j = 1; j <= prime[0] && prime[j] <= maxn / i; j++){
			prime[prime[j] * i] = true;
			if(i % prime[j] == 0) break;
		}
	}
	int n;
	while(scanf("%d", &n) == 1){
		int l = 0, r = prime[0] + 1;
		while(r - l > 1){
			int m = (l + r) / 2;
			if(prime[m] <= n) l = m;
			else r = m;
		}
		printf("%d\n", l);
	}
	return 0;
}

C++: #include <iostream>
#include <bitset>

using namespace std;

int main() {
  bitset<1000000> prime;
  int i, j, n;
  while (cin >> n) {
    prime.reset();
    for (i = 2; i <= n; i++)
      if (prime[i] == false)
	for (j = i + i; j <= n; j += i)
	  prime[j] = true;
    cout << n - prime.count() - 1 << endl;
  }
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int prime(int);
int main(){
    int a,n;
    n=0;
    while ( cin >> a){
        for (int i=2;i<a;i++){
            if (prime(i)==0){
                n++;
            }
        }
        cout << n << endl;
    }
}


int prime(int x){
    if(x==2 || x==3 || x==5 || x==7)
        return 0;
    if(x%2==0)
        return 1;
    if(x%3==0)
        return 1;
    for (int i=5;i<=sqrt(x)+1;i+=6){
        if(x%i==0 || x%(i+2)==0)
            return 1;
    }
    return 0;
}

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

int main() {
	int n;
	while(cin >> n && n != EOF) {
		bool* prime = new bool[n+1];
		int count = 0;
		for(int i = 0; i <= n; ++i) {
			prime[i] = true;
		}
		prime[0] = prime[1] = false;
		
		for(int i = 0; i <= n; ++i) {
			if(prime[i]) {
			//	cout << i << '\t';
				for(int j = 2 * i; j <= n; j += i) {
					prime[j] = false;
				}
			}
		}
		
		for(int i = 0; i <= n; ++i) {
			if(prime[i]) {
				count++;
			}
		}
		
		cout << count << endl;
		delete[] prime;
	}
}

C++: #include<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);i++)
#define loop(i,x,n) for(int i=(x);i<(n);i++)
#define all(x) (x).begin(),(x).end()
#define pb push_back
#define mp make_pair
#define fir first
#define sec second
#define ub upper_bound
#define lb lower_bound
#define int long long
using namespace std;
const int MOD=1000000007;
const int INF=1000000009;

bool Is_Prime(int n){
  if(n==1) return false;
  for(int i=2;i*i<=n;i++){
    if(n%i==0){
      return false;
    }
  }
  return true;
}
int p[100100];

signed main(){
  loop(i,1,100001){
    if(Is_Prime(i))p[i]=1;
  }
  rep(i,10001)p[i+1]+=p[i];
  int m;
  while(cin>>m){
    cout<<p[m]<<endl;
  }
  return 0;
}

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

bool f[1000000];

int main(){
	int n;
	while(cin>>n){
		memset(f,0,n*sizeof(bool));
		int l=sqrt(n);
		for(int i=2;i<=l;i++){
			for(int j=2;j*i<=n;j++){
				f[j*i]=true;
			}
		}
		int cnt=0;
		for(int i=2;i<=n;i++) if(!f[i])cnt++;
		cout<<cnt<<endl;
	}
}

C++: #include<stdio.h>
int flg[1000000];
int main(void)
{

	int a,i,j,cnt;
	while(scanf("%d",&a)!=EOF){	
		for(i=2;i<=a;i++){
		flg[i]=1;
		}
		for(i=2;i*i<=a;i++){
			if(flg[i]==1){
				j=2;
				while(i*j<=a){
				flg[i*j]=0;
//				printf("i=%d j=%d a=%d\n",i,j,a);
				j++;
				}
			}
		}
//		for(i=2;i<=10;i++)	printf("flg=%d\n",flg[i]);
		cnt=0;
		for(i=2;i<=a;i++){
//		printf("i=%d\n",i);
			if(flg[i]==1){
//			printf("i=%d\n",i);			
			cnt++;
			}
		}
		printf("%d\n",cnt);
	}	
	return 0;
}

C++: #include<iostream>
using namespace std;
int main(int argc,char* argv[]){
unsigned x[30];
int count=0;
for(int i=0;i<argc;i++){
cin>>x[i];
}
for(int i=0;i<argc;i++){
for(int j=1;j<x[i];j++){
if(x[i]%j!=0)
count++;
}
cout<<count<<endl;
count=0;
}
return 0;
}