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++: //#define _GRIBCXX_DEBUG
#include <algorithm>
#include <cctype>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iostream>
#include <iomanip>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
using namespace std;


// ?????¬???????????¬??????
#pragma region MACRO
#define P(x) cout << (x) << endl
#define p(x) cout << (x)
#define PED cout << "\n"
#define rep(i,n) for(int i=0; i<(int)n; ++i)
#define REP(i,x,n) for(int i=x; i<(int)n; ++i)
#define repi(i,n) for(int i=0; i<=(int)n; ++i)
#define REPI(i,x,n) for(int i=x; i<=(int)n; ++i)
#define ILP while(true)
#define FOR(i,c) for(__typeof((c).begin())!=(c).begin(); i!=(c).end(); ++i)
#define ALL(c) (c).begin(), (c).end()
#define mp make_pair
#pragma endregion

#pragma region TYPE_DEF
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<string, string> pss;
typedef pair<string, int> psi;
typedef pair<int, string> pis;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<long> vl;
typedef vector<long long> vll;
typedef vector<string> vs;
#pragma endregion


// Effective std
#pragma region ESTD
template<typename C, typename T>
constexpr int count(C& c, T t) { return count(ALL(c), t); }
template<typename C, typename F>
constexpr int count_if(C& c, F f) { return count_if(ALL(c), f); }
template<typename C, typename T, typename U>
constexpr void replace(C& c, T t, U u) { replace(ALL(c), t, u); }
template<typename C, typename F, typename U>
constexpr void replace_if(C& c, F f, U u) { (ALL(c), f, u); }
template<typename C>
constexpr void sort(C& c) { sort(ALL(c)); }
template<typename C, typename Pred>
constexpr void sort(C& c, Pred p) { sort(ALL(c), p); }
template<typename C>
constexpr void reverse(C& c) { reverse(ALL(c)); }
#pragma endregion


// ?´???°
#pragma region PRIME
bool is_prime(unsigned n) {
  switch(n) {
    case 0:
    case 1: return false;
    case 2: return true;
  }
  if (n%2==0) return false;
  for (unsigned i=3; i*i<=n; i+=2)
    if (n%i==0) return false;
  return true;
}
#pragma endregion


// ??§??????/?°?????????????
#pragma region TRANSFORM
void mutal_tr(string &s) {
  for(int i=s.size(); i--;) {
    if(islower(s[i])) s[i] = toupper(s[i]);
    else if (isupper(s[i])) s[i] = tolower(s[i]);
  }
}
void to_upper(string &s) { for(int i=s.size(); i--;) s[i] = toupper(s[i]); }
void to_lower(string &s) { for(int i=s.size(); i--;) s[i] = tolower(s[i]); }
#pragma endregion


// ??????
#pragma region SET
template<class T>
set<T> intersection(const set<T>& sa, const set<T>& sb) {
  set<T> ret;
  for(T a : sa) if(sb.find(a) != sb.end()) ret.insert(a);
  return ret;
}
#pragma endregion

// Union Find
#pragma region UF
struct UnionFind {
  vector<int> data;
  UnionFind(int size) : data(size, -1) {}
  bool union_set(int x, int y) {
    x = root(x); y = root(y);
    if(x!=y) {
      if(data[y] < data[x]) swap(x, y);
      data[x] += data[y]; data[y] = x;
    }
    return x != y;
  }
  bool find_set(int x, int y) { return root(x) == root(y); }
  int root(int x) { return data[x] < 0 ? x : data[x] = root(data[x]); }
  int size(int x) { return -data[root(x)]; }
};
#pragma endregion


// ?????°
#pragma region CONST_VAL
#define PI (2*acos(0.0))
#define EPS (1e-9)
#define MOD (int)(1e9+7)
#pragma endregion

int main()
{
    ll n;
    while(cin>>n) {
        bool isPrime[n];
        int ans  = 0;
        rep(i,n) isPrime[i] = true;
        isPrime[0] = isPrime[1] = false;
        rep(i,n) {
            if(isPrime[i])
                for(int j=i+i;j<n;j+=i) isPrime[j] = false;
        }
        REP(i,2,n) if(isPrime[i]) ans++;
        P(ans);
    }
    return 0;
}

C++: #include<stdio.h>



int main(){
	int n,i,j;
	bool c[1000000];
	int count;


	while(scanf("%d",&n)!=EOF){
		count=0;
		c[0]=c[1]=0;
		for(i=2;i<=n;i++){
			c[i]=1;
		}
		
		for(i=0;i<=n;i++){
			if(c[i]==1){
				for(j=i+1;j<=n;j++){
					if(j%i==0)c[j]=0;
				}
			}
		}


		for(i=0;i<=n;i++){
			if(c[i]==1){
				count++;
			}
		}

		printf("%d",count);


	}





	return 0;
}

C++: #include<iostream>
using namespace std;
int main()
{
	int n,S;
while(cin>>n)
{
	S=0;
	if(n==1)
	{
		cout<<0<<endl;
	}
	else if(n>=2)
	{
		S++;
	}
	else if
	{
	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<stdio.h>
#include<Math.h>

int main(){
	int n;
	int a[100000];
	int b;
	int i, j, p;
	int c[30];
	p = 0;
	while (scanf("%d", &n) != EOF){
		b = (int)pow((double)n, 0.5);
		c[p] = 0;
		a[0] = -1;
		for (i = 2; i <= b; i++){
			for (j = 2; j <= n / i; j++){
				a[i*j - 1] = -1;
			}
		}
		for (i = 1; i <= n; i++){
			if (a[i - 1] != -1){ c[p]++; }
			a[i - 1] = 0;
		}
		p++;
	}
	for (i = 0; i < p; i++){
		printf("%d\n", c[i]);
	}
	return 0;
}

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

using namespace std;

int main(void){
	long n;
	int a;
	while(scanf("%d", &n) != EOF){
		bool p[1000001];
		for(int i = 2; i <= n; i++)
			p[i] = true;
		for(int i = 2; i < sqrt(n); i++){
			if(p[i]){
				for(int j = 0; i*(j+2) <= n; j++){
					p[i*(j+2)] = false;
				}
			}
		}
		int count = 0;
		for(int i = 2; i <= n; i++){
			if(p[i]) count++;
		}
		cout << count << endl;
	}
	return 0;
}

C++: #include <stdio.h>

int main(int argc, const char * argv[])
{
    int i, j, k, n, count;
    int a[999999];
    
    count = 0;
    
    while(scanf("%d",&n) != EOF){
        if (count == 0) {
            if (n > 1) {
                a[count++] = 2;
                if (n > 2) {
                    a[count++] = 3;
                    for(i=5; i<=n; i+=2){
                        k=0;
                        for(j=0; a[j]*a[j] <= i; j++){
                            if(i%a[j] == 0) {
                                k = 1;
                                break;
                            }
                        }
                        if (k == 0) {
                            a[count++] = i;
                        }
                    }
                }
            }
            printf("%d\n",count);
        } else {
            if (n <= a[count - 1]) {
                for (i=count - 1; n < a[i]; i--);
                printf("%d\n", i+1);
            } else {
                if (a[count-1] == 2) {
                    a[count++] = 3;
                    printf("%d\n",count);
                } else {
                    for(i=a[count - 1]+2; i<=n; i+=2){
                        k=0;
                        for(j=0; a[j]*a[j] <= i; j++){
                            if(i%a[j] == 0) {
                                k = 1;
                                break;
                            }
                        }
                        if (k == 0) {
                            a[count++] = i;
                        }
                    }
                    printf("%d\n", count);
                }
            }
        }
        for (i=0; i<count; i++) printf("%d ",a[i]);
        printf("\n");
    }
    return 0;
}