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: MAX = 999999
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(n):
  if n == 2:return True
  if n % 2 == 0:return False
  for i in range(3,int(n**0.5)+1,2):
    if n % i == 0:return False
  return True

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

def prime_list(n):
  result = []
  if n >= 2:result.append(2)
  for i in range(3,n+1,2):
    if is_prime(i):result.append(i)
  return result

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

Python: prime=[2]
for i in range(3,1000000,2):
    primeq=True
    for p in prime:
        if i%p==0:
            primeq=False
            break
        if i<p*p:break
    if primeq:prime.append(i)
while True:
    try:
        n=int(input())
        ans=0
        for p in prime:
            if p>n:break
            ans+=1
        print(ans)
    except:break

Python: import itertools, functools, math, sys

def prime_stream():
    stream = itertools.count(2)
    sieve = lambda x, y: x % y != 0

    while True:
        prime = next(stream)
        stream = filter(functools.partial(sieve, y=prime), stream)
        yield prime

for line in sys.stdin:
    primes = prime_stream()
    num = int(line)
    count = 0
    for i in range(num):
        if next(primes) > num:
            break
        count += 1
    print(count)

Python: primes = [0, 0] + [1]*999999
for i in range(2, 1001):
    if primes[i]:
        for j in range(i*i, 1000001, i):
            primes[j] = 0
while True:
    try:
        data = int(input())
    except:
        break
    print(sum(1 for i in range(data, 1, -1) if primes[i]))

Python: def sieve(n):
	p=[True]*(n+1)
	p[0]=p[1]=False
	for i in range(2,n+1,2):
		if p[i]==True:
			for j in range(i*i,n+1,i):
				p[j]=False
	return p

def solve(n):
		if n<2:return 0
		c=1
		for i in range(3,n+1,2):
			if p[i]==1:
				c+=1
		return c

p=sieve(1e6)
while True:
	try:
		n = int(input())
		print(solve(n))
	except EOFError:
		break

Python: prime=[False]*1000000
np=[0]*1000000
for i in range(2,1000):
    if not prime[i]:
        for j in range(i*2,1000000,i):
            prime[j]=True

while True:
    try:
        n=int(input())
    except:
        break
    print(sum(prime[2:n+1]))

Python: import sys

num = []
for i in sys.stdin:
    num.append(int(i))

lst = [i for i in range(2, max(num))]
for i in lst[::-1]:
    for j in range(2, i):
        if i % j == 0:
            lst.pop(lst.index(i))
            break

for i in num:
    for j in lst:
        if i < j:
            print(lst.index(j))
            break

Python: import math
b=[]
b.append(1)
b.append(2)
count=1
s=0
while s<30:
    try:
        x=int(input())
        if x>b[count]:
            for i in range(b[count]+1,x+1):
                count1=0
                
                for j in range(2,int(math.sqrt(i))+1):
                    if i%j==0:
                        count1=1
                        break
                if count1==0:
                    count+=1
                    b.append(i)
            print(count)
        else:
            for i in range(count+1):
                if b[i]>x:
                    print(i-1)
                    break
                elif b[i]==x:
                    print(i)
                    break
        s+=1
    except EOFError:
        break

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

# n??\???????´???°?????°???????????¢??°
def prime(n):
    l = range(0,n+1)
    i,k = 0,2
    result = 1

    if n >= 3:
        prime_nums = [2, 3]
    else:
        prime_nums = []

    while i <= n:
        while l[i] > k:
            result = l[i]%k
            k = k+1
            if result == 0:
                break
            if (l[i]-1 == k) and (result != 0):
                prime_nums.append(l[i])

        i, k = i+1, 2

    #return prime_nums
    return len(prime_nums)

array = []
#for i in sys.stdin:
#    array.append(int(i))

#l = len(array)
#for i in array:
#    psum = prime(i)
#    print(psum)
    
for i in sys.stdin:
    psum = prime(int(i))
    print(psum)

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

        num = int(num.strip())
        if prim_vals.get(num) is not None:
            cnt = prim_vals.get(num)
        else:
            #print('num:', num)
            if num == 1:
                cnt = 0
            else:
                cnt = 0
                #for i in range(3, num + 1, 2):
                if num % 2 == 0:
                    start_num = num -1
                else:
                    start_num = num
                    
                for i in range(start_num, 0, -2):
                    #print('i:', i)
                    if prim_vals.get(i) is not None:
                        cnt += prim_vals.get(i)
                        #cnt -= 1
                        break
                    
                    if is_prime(i):
                        cnt += 1
                        
                #cnt += 1             # 2??????????¶????
                prim_vals[num] = cnt # ????????°?????§????´???°????????°????????????(2?????????)

        print(cnt)

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