Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

Python: # ?´???°?????°?????¨?????????????????°??????

import sys
import math

#prim_no = {2: True}                    # ????????°????´???°??§??????????????????????????????

def is_prime(no):
    if no == 2 or no == 1:
        return True
    
    if no % 2 == 0:
        return False

    # if prim_no.get(no) is not None:
    #     return prim_no.get(no)

    max_check = int(math.sqrt(no))
    for i in range(3, max_check+1, 2):
        if no % i == 0:
            # prim_no[no] = False
            return False

    # prim_no[no] = True
    return True

def main():
    prim_vals = {}                         # ????????°?????§????´???°????????°
    num_data = []                          # ?????????????????????
    
    while True:
        num = sys.stdin.readline()
        if num is None or num.strip() == '':
            break

        num = int(num.strip())
        num_data.append(num)

    sorted_num_data = sorted(num_data)
    prim_num = {}
    
    for num in sorted_num_data:
        if prim_vals.get(num) is not None:
            cnt = prim_vals.get(num)
        else:
            if num == 1:
                cnt = 0
            else:
                cnt = 0

                if num % 2 == 0:
                    start_num = num -1
                else:
                    start_num = num
                    
                for i in range(start_num, 0, -2):
                    if prim_vals.get(i) is not None:
                        cnt += prim_vals.get(i)
                        break
                    
                    if is_prime(i):
                        cnt += 1
                        
                prim_vals[num] = cnt # ????????°?????§????´???°????????°????????????
        prim_num[num] = cnt

    for num in num_data:
        print(prim_num[num])

if __name__ == '__main__':
    main()
    #print(prim_no)

Python: L = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757, 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863, 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997]

def is_prime_2(n):
  a = int(n ** 0.5)
  for i in L:
    if i > a:return True
    if n % i == 0:return False
  return True

def prime_count(n):
  result = 0
  if n >= 2:result+=1
  for i in range(3,n+1,2):
    if is_prime_2(i):result+=1
  return result

while 1:
  print(prime_count(int(input())))

Python: # -*- coding: utf-8 -*-

import sys
import os
import math



def eratosthenes(n):
    lst = [i for i in range(2, n+1)]
    primes = []
    sqrt_n = n ** 0.5

    while True:
        first = lst.pop(0)
        if first < sqrt_n:
            primes.append(first)
            new_lst = []
            for v in lst:
                if v % first != 0:
                    new_lst.append(v)
            lst = new_lst
        else:
            # ??????
            return primes + [first] + lst

for s in sys.stdin:
    n = int(s)
    lst = eratosthenes(n)
    print(len(lst))

Python: import sys
m=500000;a=[1]*m
for i in range(3,999,2):
 if a[i//2]:a[i*i//2::i]=[0]*len(a[i*i//2::i])
p=[]
for i in range(m*2):p.append(i-1 if i<4 else p[i-1]+[0,a[i//2]][i%2])
for e in sys.stdin:print(p[int(e)])

Python: import sys
import math


LIMIT = 1000000
p = 2
pList = [True] * (LIMIT + 1)


while p ** 2 <= LIMIT:
    if(pList[p]):
        for i in range(p * 2, LIMIT + 1 , p):
            pList[i] = False
    p += 1

# print(pList)


lines = str(sys.stdin.read()).strip().split("\n")

for line in lines:
    line = int(line)
    count = 0
    for i in range(2, line + 1):
        if pList[i]:
            count += 1
    print(count)

Python: # coding: utf-8
# Here your code !
import math
 
def is_prime_num(in_num):
    cnt = 0
    if in_num == 2:
        return True
    elif in_num <= 1:
        return False
    elif in_num % 2 == 0:
        return False
    else:
        if pow(2, in_num-1, in_num) == 1:
            max_num = int(math.sqrt(in_num)+1)
            for j in range(3, max_num, 2):
                if in_num % j == 0:
                    cnt = cnt + 1
                    break
                         
            if cnt == 0:
                #??´????°
                return True
            else:
               return False
        else:
            return False

while True:
    try:
        n = int(input())
        prime_cnt = 0
        if n >= 2:
            #?????????2???????????????
            prime_cnt = prime_cnt + 1

        for i in range(3, n+1, 2):
            if is_prime_num(i):
                prime_cnt = prime_cnt + 1
                 
 
        print(str(prime_cnt))
    except:
        break

Python: def isPrime(x):
    if x==2:
        return True
    if x<2 or x%2==0:
        return False
    if pow(2,x-1,x)==1:
        return True
    else:
        return False
while 1:
    try:
        n=input()
        ct=0
        for i in xrange(n+1):
            if isPrime(i):
                ct+=1
        print(ct)
    except:
        break

Python: def get_prime_set(ub):
    from itertools import chain
    from math import sqrt

    if ub < 4:
        return ({}, {}, {2}, {2, 3})[ub]

    ub, ub_sqrt = ub+1, int(sqrt(ub))+1
    primes = {2, 3} | set(chain(range(5, ub, 6), range(7, ub, 6)))
    du = primes.difference_update
    for n in chain(range(5, ub_sqrt, 6), range(7, ub_sqrt, 6)):
        if n in primes:
            du(range(n*3, ub, n*2))

    return primes


import sys
from bisect import bisect_right
primes = list(get_prime_set(999999))
print(*(bisect_right(primes, int(n)) for n in sys.stdin), sep="\n")

Python: from math import sqrt,floor
from sys import stdin

def cntPrime(n):
    mx = sqrt(n)
    if n % 2 == 0:
      l = [0, 1] + [1, 0] * (int(n/2) - 1)
    else:
      l = [0, 1] + [1, 0] * (int(n/2) - 1) + [1]
    c = 3
    while c < mx:
        for k in range(c*2, n+1, c):
            l[k-1] = 0
        c = l[(c+1):].index(1) + c +2
        while c % 2 == 0:
            c = l[(c+1):].index(1) + c +2
    return l
t = cntPrime(1000000)
for n in stdin:
    n = int(n)
    print(sum(t[:n]))

Python: import math
def kosu(n):
    l=[]
    sosu = [True] * (n + 1)
    sosu[0]= False
    sosu[1]= False
    for i in range(2,math.floor(math.sqrt(n))+1):
        if not sosu[i]:
            continue
        for j in range(i*2,n+1,i):
            sosu[j] = False
    for i in range(len(sosu)):
        if sosu[i]:
            l.append(i)
    return(l)
    
while True:
    try:
        n = int(input())
    except EOFError:
        break
    print(len(kosu(n)))