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<bits/stdc++.h>
#define rep(i,n) for(int i=0; i<n; i++)
using namespace std;
int main(){
  int n;
  while(cin >> n){
    int cnt=0,res=0;
    for(int i=3; i<=n; i+=2){
      for(int j=1; j<=i; j++){
        if(i%j == 0) cnt++;
        if(cnt>2) break;
      }
      if(cnt==2) res++;
      cnt = 0;
    }
    cout << res+1 << endl;
  }
  return 0;
}

C++: #include<cstdio>
#include<iostream>
using namespace std;
int prime[1000000];
int main(){
    int n,ans=0;
    for(int i=0;i<1000000;i++)
        prime[i]=1;
    prime[1]=0;
    prime[2]=1;
    for(int i=1;i<1001;i++){
        if(prime[i]==1){
            for(int j=i+1;j<1000000;j++){
                if(j%i==0)
                    prime[j]=0;
            }
        }
    }
    scanf("%d",&n);
    for(int i=1;i<n;i++){
        if(prime[i]==1)
        ans++;
    }
    printf("%d",ans);
    return 0;
}

C++: /*http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0009&lang=jp*/

#include <iostream>

#define MAX 999999

using namespace std;

int main()
{
    int n;
    int num_prime[MAX];
    bool prime_flag[MAX];

    for (int i = 1; i < MAX; ++i) {
        if(!prime_flag[i]){
            for (int j = 2; (i+1)*j-1 < MAX; ++j) {
                prime_flag[(i+1)*j-1]= true;
            }
        }
        num_prime[i]=num_prime[i-1]+!prime_flag[i];
    }
    while (cin>>n){
        cout<<num_prime[n-1]<<endl;
    }
    return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
int main(){
    int n;
    while(cin >> n){
        bool prime[1000000] = {false};
        // falseで初期化
        prime[1] = true;
        prime[2] = false;
        int count = 0;
        
        // エラトステネスのふるい
        for(int j = 2; j * j <= n ; j++){
            if(!prime[j]){
                for (int k = 2; k*j <= n;k++){
                   prime[j*k] = true;
                }
            }
        }
        // falseだと素数
        for(int i = 1;i<=n;i++){
            if(!prime[i]) count = count +1;
        }
        cout << count << endl;
    }
    return 0;
}

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

bool IsPrime(int i){
    if(i < 2) return false;
    else if(i == 2) return true;
    else if(i % 2 == 0) return false;

    double sqrtNum = sqrt(i);
    for(int j = 3; j <= sqrtNum; j += 2){
        if(i % j == 0){
            return false;
        }
    }

    return true;
}

int main(){
    int n;
    while(cin >> n){
        int k = 0;
        for(int i = 0; i <= n; ++i){
           if(IsPrime(i) == true){
               k++;
           }else ;
        }
        cout << k << endl;
    }
}

C++: #include <iostream>
#include <cmath>
using namespace std;
bool isprime(int& num)
{
    static int c;
    for(c=2; c < sqrt(num)+1; c++)
        if(num % c ==0) return false;
    return true;
}

int main()
{
    int ans = 0;
    int buff;
    while(cin >> buff);
    {
        for(int d =2; d <= buff; d++)
            if(isprime(d)) ans++;
        cout << ans << " ";
        ans  = 0;
    }
    cout << endl;
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <cstdio>
#include <string>
#define rep(x,to) for(x=0;x<to;x++)
#define rep2(x,from,to) for(x=from;x<to;x++)

using namespace std;


int main(void){

int num,i,cnt;


	while(cin >> num){
		cnt=1;
		for(i=3;i <= num;i+=2){
			bool swa = true;
			for(int j=3; j*j <= i;j+=2){
				if(i % j == 0){
					swa=false;
					continue;
				}

			}
			if( swa == true ){
				cnt++;
//				cout << i << " ";
			}
		}
		cout << cnt <<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <math.h>
#include <vector>
using namespace std;
int main(){
	int n;
	while(cin>>n){
		bool prime[1000000];
		for(int l=2;l<=n;l++){
			prime[l] =true;
		}
		for(int i=2;i<=n;i++){
			if(prime[i]==true){
				for(int j=i*2;j<=n;j+=i){
					prime[j]=false;
				}
			}
		}
		int count=0;
		for(int k=2;k<=n;k++){
			if(prime[k]==true) count++;
		}
		cout<<count<<endl;
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int s[80000],m[1000000],i,j,n=0;
int main() {
	for (i=2; i<1000000 ; i++) {
		if (m[i]==0) {
			s[n++]=i;
			for (j=i; j<1000000; j+=i) m[j]=1;
		}
	}
	while(cin >> i) {
		for (j=0;s[j]<=i && j<n;j++) ;
		cout << j << endl;
	}
return 0;
}

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

#pragma warning(disable:4996)

int main()
{
	int DataSet,nCnt,nCnt2,In,Total,Count;

	Total = 0;
	DataSet = 0;


	for (DataSet = 0; DataSet <= 30; DataSet++)
	{
		scanf("%d", &In);
		Total = 0;
		for (nCnt = 2; nCnt <= In; nCnt++)
		{
			Count = 0;
			for (nCnt2 = nCnt; nCnt2 >= 2; nCnt2--)
			{
				if (nCnt%nCnt2 == 0)
				{
					if (nCnt != nCnt2)
					{
						Count++;
						break;
					}
				}
			}
			if (Count == 0)
				Total++;
		}
		printf("%d\n", Total);
	}

	return 0;
}

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

using namespace std;

int main()
{
    int n;
    const int max_index = 1000000;
    int prime[max_index];

    prime[0] = 0;
    for (int i = 1; i < max_index; i++) 
        prime[i] = 1;

    for (int i = 1; i < (int)sqrt(max_index); i++) {
        if (prime[i] == 1)
            for (int j = i + 1; (i + 1) * j <= max_index; j++)
                prime[(i + 1) * j - 1] = 0;
    }

    int tmp = 0;
    for (int i = 0; i < max_index; i++) 
        (prime[i] == 0) ? (prime[i] = tmp) : prime[i] = ++tmp;
    prime[0] = 0;

    while (cin >> n) 
        cout << prime[n - 1] << endl;
    
    return 0;
}

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

using namespace std;

int main()
{ 
	int n;
	while (cin >> n)
	{
		vector<bool> a(n + 1, true);
		int cnt = n == 0 ? 0 : n - 1;

		// Sieve of Eratosthenes
		for (int i = 2; i*i <= n; ++i)
			if (a[i])
				for (int j = 0; i * (j + 2) <= n; ++j)
					if (a[i * (j + 2)]) a[i * (j + 2)] = false, cnt--;

		cout << cnt << endl;
	}

	return 0;
}

C++: #include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<ctime>
#include<cctype>
#include<climits>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<memory>
#include<functional>

#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define llong long long
#define pb(a) push_back(a)
#define INF 999999999
using namespace std;

typedef vector<int> vi;
typedef vector<long long> vl;
typedef vector<double> vd;
typedef pair<int, int> pii;
typedef pair<long, long> pll;
typedef long long ll;

int dy[] = { 0, 0, 1, -1, 0 };
int dx[] = { 1, -1, 0, 0, 0 };


int main(){
	ios::sync_with_stdio(false);

	int n;
	int count;
	int nmax = 1000000;
	vector<bool> flag(nmax+1, true);

	flag[0] = !flag[0];
	flag[1] = !flag[1];

	for (int i = 2; i <= sqrt(nmax) + 1; i++){
		if (flag[i]){
			for (int j = i * 2; j <= nmax; j += i){
				flag[j] = false;
			}
		}
	}
	
	while (cin >> n){
		count = 0;
		for (int i = 0; i <= n; i++){
			if (flag[i])
				count++;
		}

	printf("%d\n", count);
	}

}