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++: /////////////////////////////////////////////////////////
//
//  c/temp.cpp file
//  Last Updated: 2018-10-23 ...Maybe
//
//    I hope you adding this code to the setting file
//    alias g++='g++ -std=c++1y -DDEBUG_LOCAL'
//
/////////////////////////////////////////////////////////

#include <bits/stdc++.h>

using namespace std;

#define fi first
#define se second
#define mp make_pair
#define pb push_back

//#define int long long

#define _overload3(_1,_2,_3,name,...) name
#define _REP(i,n) REAP(i,0,n)
#define REAP(i,a,b) for(int i=int(a);i<int(b);++i)
#define REP(...) _overload3(__VA_ARGS__,REAP,_REP,)(__VA_ARGS__)

#define _REPR(i,n) REAPR(i,n,0)
#define REAPR(i,a,b) for(int i=int(a-1);i>=int(b);--i)
#define REPR(...) _overload3(__VA_ARGS__,REAPR,_REPR,)(__VA_ARGS__)

#define ALL(a) a.begin(),a.end()
#define rALL(a) a.rbegin(),a.rend()

#define coutALL(a) {int loop_coutALL=0;for(auto e:a) cout<<(loop_coutALL++?" ":"")<<e; cout<<endl;}

#define coutYN(a) cout<<((a)?"YES":"NO")<<endl;
#define coutYn(a) cout<<((a)?"Yes":"No")<<endl;
#define coutyn(a) cout<<((a)?"yes":"no")<<endl;

#define pcnt __builtin_popcount
#define buli(x) __builtin_popcountll(x)

const int INF=1145141919;
//const long long INF=114514191911451419;
const int MOD=(int)1e9+7;
const double EPS=1e-12;
const int dx[]={1,0,-1,0},dy[]={0,1,0,-1};
//const int dx[]={-1,-1,-1,0,1,1,1,0},dy[]={-1,0,1,1,1,0,-1,-1};

using vi=vector<int>;
using vvi=vector<vi>;
using vs=vector<string>;

typedef pair<int,int> pii;
typedef pair<int,string> pis;
typedef pair<string,int> psi;
typedef pair<string,string> pss;

typedef long long ll;

template<typename T> istream &operator >> (istream &is, vector<T> &v){for(T &x:v) is>>x;return is;}
template<typename T> ostream &operator << (ostream &os, vector<T> &v){int i=0;for(T &x:v) os<<(i++?" ":"")<<x;return os;}

ll rev(ll n){ll x=0;for(;n>0;){x=x*10+n%10;n/=10;}return x;}

ll upper(ll n,ll m){return (n+m-1)/m;};
ll rounding(ll n){return (long double)n+0.5;};

bool inG(ll x,ll n){return 0<=x && 0<n;}
bool outG(ll x,ll n){return x<0 || n<=x;}

inline int qp(int a,ll b){if(!b)return 1;int ans=1;do{if(b&1)ans=1ll*ans*a;a=1ll*a*a;}while(b>>=1);return ans;}
inline int qp(int a,ll b,int mo){if(!b)return 1;int ans=1;do{if(b&1)ans=1ll*ans*a%mo;a=1ll*a*a%mo;}while(b>>=1);return ans;}

inline ll fac(ll k,ll n){ll a=1;for(int i=0;i<n;i++)a*=k--;return a;}
inline ll fac(ll k,ll n,int mo){ll a=1;for(int i=0;i<n;i++)a*=k--,a%=mo;return a;}

inline int dsum(ll n){int a=0;for(;n;n/=10)a+=n%10;return a;}

struct Arithmetic{Arithmetic(){cin.tie(0);ios::sync_with_stdio(0);cout<<fixed<<setprecision(20);}};

//#define DEBUG_LOCAL

#ifdef DEBUG_LOCAL
template<typename T> void deb(T a){cerr<<"deb: "<<a<<"です"<<endl;}
#define debl {cerr<<"debug: "<<__LINE__<<"行目だ"<<endl;}
void what_cr(){cout<<__GCC_ATOMIC_CHAR16_T_LOCK_FREE<<" ←？"<<endl;}
//ori_func S

//ori_func E
#else
template<typename T>void deb(T a){}
#define debl
void what_cr(){}void t_t(){}
#endif


const int MAX_primear=1e7; // 1e8=x, 1e7=? 1e6=o
int primear[MAX_primear+1]={};
vector <int> prime;
void eratos_sieve(){
  for(int i=2;i<=MAX_primear;i++){
    if(!primear[i]){
      prime.push_back(i);
      primear[i]=2;
      for(int j=i*2;j<=MAX_primear;j+=i) primear[j]=1;
    }
  }
}

signed main(){

  Arithmetic Exception;

  eratos_sieve();

  for(int n;cin>>n;){

    int ans=0;
    REP(i,prime.size()) ans += prime[i]<= n;

    cout<<ans<<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 = x; i > 1; i--) {
			if (prime(i))count++;
		}
		cout << count << endl;
	}
	return 0;
}

bool prime(int n) {
	for (int i = n - 1; i > 1; i--) {
		if (n % i == 0)return false;
	}
	return true;
}

C++: #include <stdio.h>
int main(){
	int i,j,n,num;
	bool flag=true;
	while(scanf("%d",&n)!=EOF){
		num=0;
		for(i=3;i<n+1;i++){
			for(j=2;j<i;j++){
				if(i%j==0){
					flag=false;
					break;
				}
			}
		    if(flag) num++;
			else flag =true;
		}
		num++;
		printf("%d\n",num);
	}
}

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

bool isprime(int n){
	if(n < 2)return false;
	
	for(int i=2;i*i<=n;i++)
		if( n%i == 0)return false;
	return true;
}
int main(){
	int n;
	bool memo[1000000] = {false};
	for(int i=0;i<1000000;i++){
		memo[i] = isprime(i);
	}
	while(cin >> n){
		int output = 0;
		for(int i=0;i<=n;i++){
			if(memo[i])output++;
		}
		cout << output << endl;
	}
}

C++: #include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < (int)(n); i++)

using namespace std;

typedef long long ll;
typedef pair<int, int> P;
typedef pair<ll, ll> LP;

const int MOD = 1000000007;
const int INF = 1000000000;
const ll LINF = 1000000000000000000L;
const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};
const int ddx[] = {1, 1, 0, -1, -1, -1, 0, 1};
const int ddy[] = {0, 1, 1, 1, 0, -1, -1, -1};

int extgcd(int a, int b, int& x, int&y) {
    int d = a;
    if(b != 0) {
        d = extgcd(b, a % b, y, x);
        y -= (a / b) * x;
    }else {
        x = 1;
        y = 0;
    }
    return d;
}

int inv(int a) {
    int x, y;
    extgcd(a, MOD, x, y);
    return (x + MOD) % MOD;
}

const int MAX_N = 1000000;
bool ifprime[MAX_N + 1];
bool used[MAX_N + 1];
int sum [MAX_N + 1];

int main() {
    fill(used, used + MAX_N + 1, false);
    fill(ifprime, ifprime + MAX_N + 1, false);
    for(int i = 2; i <= MAX_N; i++) {
        if(!used[i]) {
            used[i] = true;
            ifprime[i] = true;
            for(int j = 1; i * j <= MAX_N; j++) {
                used[i * j] = true;
            }
        }
    }

    sum[0] = 0;
    rep(i, MAX_N) {
        sum[i + 1] = sum[i];
        if(ifprime[i + 1]) sum[i + 1]++;
    }

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

C++: #include<iostream>
#include<vector>
using namespace std;
 int main(){
    int n,cnt;
  while(cin >> n){
    cnt=0;
    vector<bool>m(n+1,true);
    for(int i=2;i*i<=n;i++)
         if(m[i]) for(int j=i+i;j<=n;j+=i)  m[j]=false;        
    for(int i=2;i<=n;i++)
            if(m[i]) cnt++;
    cout << cnt << endl;  
  } 
  return 0;
}

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

using namespace std;

int get_numprime( int num );

int main(void)
{
	int num;

	while(1)
	{
		cin >> num;
		if( cin.eof() ) break;
		cout << get_numprime( num ) << endl;
	}

	return 0;
}

int get_numprime( int num )
{
	int numprime = 1; // 2はすでにカウント

	for( int i = 3 ; i <= num ; i+=2 )
	{
		int flag = 1;
		for( int j = 3 ; j <= sqrt((double)i) ; j+=2 )
		{
			if( i%j == 0 )
			{
				flag = 0;
				break;
			}
		}
		if( flag == 1 ) numprime++;
	}

	return numprime;
}

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 || 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;
}