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>
#include <cmath>
#include <algorithm>
#include <map>
#include <string>
#include <vector>
#define FOR(i, a, b)  for(int i=a; i<b; ++i)
#define rFOR(i, a, b) for(int i=a-1; i>=b; --i)
#define REP(i, n) for(int i=0; i<n; ++i)
#define rREP(i, n) for(int i=n-1; i>=0; --i)
#define all(a)  a.begin(), a.end()
using namespace std;

int main(){
  int n;
  while(cin >> n){
    int ans=0;
    vector<int> vec;
    vec.push_back(2);
    for(int i = 3; i<=n; i+=2){
      bool isPrimeN = true;
      for(auto&& num : vec){
        if(i%num==0){
           false;
        }
      }
      for(int j=3; j<=floor(sqrt(i)); j++){
        if(i%j==0){
          isPrimeN = false;
          break;
        }
      }
      if(isPrimeN){
        vec.push_back(i);
      }
    }
    cout << vec.size() << endl;;
  }
  return 0;
}

C++: #include <iostream>
#include <algorithm>
 
using namespace std;
 
#define rep(i, n) for(int i=0; i<(n); ++i)

bool isPrime[999999];

int main(void){
    int n;
    while(cin >> n) {
        rep(i, n+1) isPrime[i] = true;
        for(int i=2; i*i <= n; ++i) {
            if(isPrime[i]) {
                for(int j=2; i*j <= n; ++j) {
                    isPrime[i*j] = false;
                }
            }
        }
        int ans = 0;
        for(int i=2; i<=n; ++i) {
            if(isPrime[i]) ans++;
        }
        cout << ans << endl;
    }
}

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

int main(){
int unti[100001] = {0};
fill(unti,unti + 100001,0);
for(int n = 2;n < 100001;n++)
{
if(unti[n] == 0)
{
for(int a = 2;(a * n) < 100001;a++)
{
	if((a * n  ) < 100001 ) unti[a * n]++;
}
}
}
int b;
while(cin >> b )
{
int cnt = (count(unti,unti+b+1,0) - 2);
cout << cnt ;
	}


}

C++: #include<iostream>
using namespace std;
int main()
{
	int n,S;
while(cin>>n)
{
	S=0;
	if(n==1)
	{
		cout<<0<<endl;
	}
	for(int i=2;i<=n;i++)
	{
		for(int j=i-1;j>1;j--)
		{
			if(i%j==0)
			{
				break;
			}
			if(j==2)
		{
			S++;
		}
		}	
			
	}
	cout<<S<<endl;
}
return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

int p[1000010];

int main(){
	int n;
	
	for(int i = 2 ; i <= 1000000 ; i++){
		p[i] = 1;
		for(int j = 2 ; j*j <= i ; j++){
			if( i % j == 0 ){
				p[i] = 0;
				break;
			}
		}
		p[i] += p[i-1];
	}
	
	while(cin >> n){
		cout << p[n] << endl;
	}
}

C++: #include<iostream>

#define bound 1000000

int main()
{
  bool is_prime[bound] = {};
  for (int i = 2; i < bound; i++)
    is_prime[i] = true;
  for (int prime = 2; prime < bound; ++prime)
    {
      if (!is_prime[prime]) continue;
      for (int i = prime * 2; i < bound; i += prime)
	is_prime[i] = false;
    }

  int table[bound] = {};
  for (int i = 0; i < bound; ++i)
      table[i] = table[i - 1] + is_prime[i];

  int n;
  while(std::cin >> n)
      std::cout << table[n] << std::endl;

  return 0;
}

C++: #include<iostream>
#include<math.h>
using namespace std;
int main(){
	int i,j,number,count=0,hantei=0;
	while(cin>>number){
		count=0;
		for(i=2;i<number;i++){
			hantei=0;
			for(j=0;j<sqrt(i);j++){
				if(i%j==0){
					hantei++;
				}
			}
			if(hantei==0){
				count++;
			}
		}
		cout<<count<<endl;
	}
	return 0;
}

C++: #include <iostream>

bool prime(int n);

int main(void) {
	using namespace std;
	int x;
	int count;
	while (cin >> x) {
		count = 0;
		for (int i = 2; i <= x; i++) {
			if (prime(i))count++;
		}
		cout << count << endl;
	}
	return 0;
}

bool prime(int n) {
	static int flag = 0; 
	static int count = 0;
	static int list[78498];

	if (flag == 1) {
		for (int j = 0; j < count; j++) {
			if (n % list[j] == 0 ) {
				if (n == list[j]) {
					return true;
				}
				
				return false;
				
				
			}
		}
	}

	
	flag = 1;
	list[count] = n;
	count++;
	return true;
}

C++: #include <bits/stdc++.h>

using namespace std;

int main(){
	int i, j, n, *prime;
	while(cin >> n){
		int ans = 0;
		prime = new int[n];
		for(i = 0; i < n; ++i) prime[i] = 1;
		prime[0] = 0;
		for(i = 1; i < sqrt(n); ++i){
			if(prime[i] == 1){
				for(j = (i + 1); (i + 1) * j <= n; ++j){
					prime[(i + 1) * j - 1] = 0;
				}
			}
		}
		for(i = 0; i < n; ++i){
			if(prime[i] == 1) ++ans;
		}
		cout << ans << endl;
	}
	return 0;
}

C++: LIMIT = 10000000
 
isPrime = [True for _ in range(LIMIT)]

isPrime[0] = isPrime[1] = False
 
for i in range(2, int(LIMIT ** 0.5)+1):
  if isPrime[i]:
    for j in range(i * i, LIMIT, i):
      isPrime[j] = False
 
try:
    while True:
        n = int(input())
        count = 0
        for i in range(n+1):
            if isPrime[i]:
                count += 1
        print(count)
except EOFError:
  pass

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

using namespace std;

int main(int argc, const char *argv[])
{
    int N=100000;
    //cin >> N;
    bool mark[100000];
    memset(mark,true,100000);

    mark[0]=mark[1]=false;
    for(int i=4;i<N;i+=2)
        mark[i]=false;
    for(int i=3;i*i<=N;i+=2)
    {
        if(mark[i]==true)
        {
            for(int j=i*i;j<N;j+=i*2)
            {
                mark[j]=false;
            }
        }
    }

    int n;
    int cnt;
    while(scanf("%d",&n)==1)
    {
        cnt=0;
        for(int i=0;i<=n;i++)
        {
            if(mark[i]==true)
                cnt++;
        }
        cout << cnt << endl;
    }




    return 0;
}

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

int main()
{
	int n;
	const int MAX = 1000000;
	int primecount[MAX];

	for (int i = 2; i < MAX; i++) {
		primecount[i] = 1;
	}

	for (int i = 2; i < MAX; i++) {
		if (primecount[i] == 1) {
			for (int j = i * 2; i < MAX; j += i) {
				primecount[j] = 0;
			}
		}
	}

	for (int i = 1; i < MAX; i++) {
		primecount[i] += primecount[i - 1];
	}

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

	return 0;
}

C++: #include <iostream>
#include <string>
#include <cstring>
#include <fstream>
#include <random>
#include <cmath>
#include <iomanip>
#include <climits>
#include <cstdlib>
#include <algorithm>
#include <vector>
#include <map>
#include <deque>
#include <map>
#include <set>
#include <unordered_set>
#include <queue>
#include <stack>
#include <list>
#include <unordered_map>
#include <bitset>
#include <sstream>
#include <new>
#include <typeinfo>
#include <iterator>
#include <cassert>

typedef long long ll;
typedef unsigned long long ull;
constexpr ll mop = 1000000007;
constexpr ll mop2 = 998244353;

using namespace std;

constexpr ll NOT_SPECIFIED_VALUE_MIN = INT64_MIN;
constexpr ll NOT_SPECIFIED_VALUE_MAX = INT64_MAX;

template <typename T>
vector<bool> eratosthenes(T n) {
	vector <bool> IsPrime;
	if (n + 1 > IsPrime.size()) {
		IsPrime.resize(n + 1, true);
	}
	IsPrime[0] = false;
	IsPrime[1] = false;

	for (T i = 2; i*i <= n; i++) {
		if (IsPrime[i]) {
			for (T j = 2; i*j <= n; j++) {
				IsPrime[i*j] = false;
			}
		}
	}
	return IsPrime;
}

int main() {
	vector<bool> isPrime = eratosthenes(1LL << 20);
	vector<ll> numPrime((1LL << 20) + 1, 0);
	ll now = 0;
	for (ll i = 1; i <= (1LL << 20); i++) {
		if (isPrime[i] == true) {
			now++;
		}
		numPrime[i] = now;
	}

	ll n;
	n = -1;
	while (true) {
		cin >> n;
		if (n == -1)break;
		cout << numPrime[n] << endl;
		n = -1;
	}


}