Go: package main

import (
    "fmt"
    "bufio"
    "os"
    "strconv"
)

func main() {
    var limit = 1000000
    var root = 1000
    var isPrime [1000000]bool
    for i := 0; i < limit; i++ {
        isPrime[i] = true
    }
    isPrime[0] = false
    isPrime[1] = false
    for i := 2; i <= root; i++ {
        if isPrime[i] {
            for j := i * i;j < limit; j+=i {
                isPrime[j] = false
            }
        }
    }
    var primeCnt [1000000]int
    var cnt = 0
    for i := 0; i < limit; i++ {
        if isPrime[i] {
            cnt++
        }
        primeCnt[i] = cnt
    }

    var sc = bufio.NewScanner(os.Stdin)
    for sc.Scan() {
        n, _ := strconv.Atoi(sc.Text())
        fmt.Println(primeCnt[n])
    }
}

C++: /*#include <stdio.h>
  int main(){
    int a,b,c,d,n,sum=0;
      while(scanf("%d",&n) != EOF){
       for(a=0;a<10;a++){
          for(b=0;b<10;b++){
             for(c=0;c<10;c++){
                for(d=0;d<10;d++){
                    if(a+b+c+d==n)
                        sum++;
                          }
                            }
                             }
                               }
                                printf("%d\n",sum);
                                 sum=0;
                                   }
return 0;
}*/#include <stdio.h>
  int main(){
   int i,j,n,hantei=0;
   int sum=0;
    while(scanf("%d",&n) != 0){
    for(i=2;i<=n;i++){
      for(j=2;j*j<=i;j++){
        if(i%j != 0 || i == j){
            hantei += 0;
               }else{
                  hantei += 1;
                    }
}
if(hantei == 0){
sum++;

}
hantei=0;
}
printf("%d\n",sum);
sum=0;
hantei = 0;
}
return 0;
}

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

#define maxPN 500000
#define PNs  1000100
int countPN = 0, countPNs = 0;
int PrimeNum[maxPN], temp;
bool PN[PNs];

bool IsPrimeNumber(int x);

void PrimeN(){
	int i, j;
	PrimeNum[0] = 2;
	PN[2] = true;PrimeNum[countPN++] = 2;
	PN[3] = true;PrimeNum[countPN++] = 3;
//	PN[5] = true;PrimeNum[countPN++] = 5;
	
	for(i = 6; i  < PNs;i+=6){
		if(IsPrimeNumber(i-1)){
			PrimeNum[countPN++] = i-1;
			PN[i-1] = true;//cout<<i-1<<endl;
		}
		if(IsPrimeNumber(i+1)){
			PrimeNum[countPN++] = i+1;
			PN[i+1] = true;//cout<<i+1<<endl;
		}
		countPNs = i+1;//cout<<i-1<<" "<<PN[i-1]<<endl;
	}
}

bool IsPrimeNumber(int x){
	//if(x == 2 || x == 3) return true;
	//if(x <= 4 || x%2 == 0 || x%3 == 0)   return false;
	//if(x < countPNs) return PN[x];
	for(int i = 0; PrimeNum[i] * PrimeNum[i] <= x; i++)
		if(x%PrimeNum[i] == 0)return false;
	return true;
}


int main(){
//	clock_t start, end;
	int n = true, counta;
//	start = clock();
	PrimeN();
	cout<<PrimeNum[countPN-1]<<" "<<countPNs<<endl;
//	cout<<clock()-start<<"ms"<<endl;
	
	while(cin>>n){
		counta = 1;
		
		
		//cout<<(IsPrimeNumber(n)?"yes":"no")<<endl;
	//	start = clock();
		//cout<<IsPrimeNumber(n)<<endl;
		for(int i = 1; i <= n; i+=2){
			if(PN[i])counta++;
		//	cout<<i<<" "<<PN[i]<<endl;
		}
		if(n == 1) counta = 0;
		cout<<counta<<endl;
	//	cout<<clock()-start<<"ms"<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
bool flag[10000001];
 
int main(){
    int n=0,ans=0;
    for(int i=0;i <= 1000000;i++){
        if(i<=1)flag[i]=false;
        else flag[i]=true;
    }
    for(int i=2;i <= 1001 ;i++){
        if(flag[i]==true){
            for(int j=i*2 ; j<=1000000 ; j=j+i){
                flag[j] = false;
            }
        }
    }
    while(cin>>n){
        ans=0;
        for(int i=0;i <= n;i++){
            if(flag[i]==true){
                ans++;
            }
        }
        cout<<ans<<endl;
    }
}

C++: #include<stdio.h>
#include<math.h>
int main(){
	int n,c,i,j,k;
	while(scanf("%d",&n)!=EOF){
		c=1;
		for(j=3;j<=n;j+=2){
			k=1;
			i=3;
			while( i <= sqrt(j) ){
				if(j%i==0){
					k=0;
					break;
				}
				i = i + 2;
			}
			if(k){
				c++;
			}
		}
		printf("%d\n",c);
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
#define MAX 1000000


int main(){
	bool work[MAX + 1];
	int num, ans = 0;

	// 初期化
	for(int i = 6; i <= MAX; i++){
		for(int j = 2; j < (i / 2); j++){
			if (i % j == 0){
				work[i] = false;
				break;
			}else{
				work[i] = true;
			}
		}
	}
	work[0] = false;
	work[1] = false;
	work[2] = true;
	work[3] = true;
	work[4] = false;
	work[5] = true;

	// 出力
	while(cin >> num){		
		for(int i = 0; i <= num; i++){
			if(work[i] == true){
				ans++;
			}
		}
		cout << ans << endl;
	}
}

C++: ///bismillah_hir_rahmanir_rahim
///GALIB-2015331528

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

int prime[1000000];
int main()
{
    int n,c;
    cin>>n;
    if(n==1){
        cout<<"0"<<endl;
        return 0;
    }

    c=1;
    for(int i=3;i<=n;i+=2){
        if(prime[i]==0){
            c++;
            for(int j=3;i*j<=n;j+=2)
                prime[i*j]=1;
        }
    }

    cout<<c<<endl;




    return 0;
}

C++: #include<stdio.h>

bool p(int a){
	if(a==2)return true;
	if(a<2||a%2==0)return false;
	for(int i=3;i*i<=a;i+=2){
		if(a%i==0)return false;
	}
	return true;
}

int main(void){
	int n,cnt=0;
	scanf("%d",&n);
	for(int i=2;i<n;i++){
		if(p(i)){
			cnt++;
		}
	}
	printf("%d\n",cnt);
	return 0;
}

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

int main() {
	// your code goes here
	int nu[30];
	int num;
	int count = 0;
	int max = 29;
	for(int x=0;x<30;x++){
		nu [x] = -1;
		cin >> nu[x];
		if (nu[x] == -1) {
			max = x;
			break;
		}
	}
	for (int y=0;y<max;y++) {
		num = nu[y];
		count = 0;
		
		for (int i = 1;i<=num;i++) {
		if(i%2==0) {
			//cout <<"2: "<<i<<endl;
			count ++;
			continue;
		}
		if(i==5||i==7) continue;
		for (int j=9;j<(int)sqrt(i)+1;j=j+2) {
			if (i%j==0) {
				count ++;
				//cout <<"i: "<<i<<" j: "<<j<<endl;
				break;
			}
		}
	}
	cout << (num-count) <<endl;
	//cout << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int prime(int);
int main(){
    int a,sum;
    int b[999999];
    b[0]=0;
    for(int i=1;i<999999;i++){
        b[i]=prime(i);
    }
    while ( cin >> a){
        sum =0;
        for (int i=0;i<a;i++){
            sum +=b[i];
        }
        cout << sum-1 << endl;
    }
}


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

C++: #include<iostream>
#include<stdio.h>
#include<math.h>
int p(long x)
{
    long i,j=sqrt(x),k=1;
    for(i=2;i<=(j);i++)
    {
        if(x%i==0){
            k=0;
            //return k;
        break;

        }
    }
    if(k==1)return 1;
    else return 2;
}
using namespace std;
int main()
{
    double a,b,c,i,n,s=0;
    i=1;

    while(i){
        if(i>=30)break;
        cin>>n;
        s=0;
    //a=sqrt(n);
    for(i=2;i<=n;i++)
    {
        if(p(i)==1){s++;}
    }
    cout<<s<<endl;
    i++;
    }


    return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <algorithm>
#include <math.h>
#include <vector>
#include <string>

using namespace std;

int n, ct = 0;
bool p[1000001];

int main(){
	for (int i = 0; i < 1000001; i++) p[i] = 1;
	for (int i = 2; i < 1000; i++){
		if (p[i] == 1){
			for (int j = 0; i*(j + 2) < 1000000; j++){
				p[i*(j + 2)] = 0;
			}
		}
	}
	while (cin >> n){
		for (int i = 2; i < n + 1; i++)
			if (p[i] == 1) ct++;
		cout << ct << endl;
		ct = 0;
	}
}