Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

func next()string{
	return sc.Text()
}

func nextInt()int{
	a, _ := strconv.Atoi(next())
	return a
}

Python: import sys

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

L2 = prime_list(MAX)

def prime_count_2(n):
  for i,v in enumerate(L2):
    if n > v:return i
  return len(L2)

for n in sys.stdin:
  print(prime_count_2(int(n)))

Python: 
prime=[False]*1000000
for i in range(2,1001):
    for j in range(i*2,1000000,i):
        prime[j]=True

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

Python: import sys

def primes(n):
    is_prime = [True] * (n + 1)
    is_prime[0] = False
    is_prime[1] = False
    for i in range(2, int(n**0.5) + 1):
        if not is_prime[i]:
            continue
        for j in range(i * 2, n + 1, i):
            is_prime[j] = False
    return [i for i in range(n + 1) if is_prime[i]]

for e in sys.stdin:
    n = int(e)
    print(len(primes(n)))

Python: import math

def Eratos(n):
	primes = [2]
	if n % 2 == 0:
		num = [2*i+1 for i in range(1,n/2)]
	else:
		num = [2*i+1 for i in range(1,(n+1)/2)]
	tmp = []

	top = 1
	while top < math.sqrt(n):
		top = num[0]
		for i in range(1,len(num)):
			if num[i] % top != 0:
				tmp.append(num[i])
		num = tmp
		tmp = []
		primes.append(top)
	for i in range(len(num)):
		primes.append(num[i])
	return primes

def list_count(list_a,n):
	cont = 0
	for i in range(len(list_a)):
		if list_a[i] > n:
			break
		else:
			cont += 1
	return cont

primes = Eratos(1000000)
while True:
	try:
		n = int(input())
		print(list_count(primes,n))
	except EOFError:
		break

Python: from math import sqrt
I_list=[]
prime_list=[]
while True:
    try:
        I_list.append(int(input()))
    except:
        break
I_max=max(I_list)
search_set={x for x in range(2,I_max+1)}
 
 
p=-1
while p<=sqrt(I_max):
    p=min(search_set)
    prime_list.append(p)
    search_set={x for x in search_set if x%p !=0 }
 
search_list=sorted(list(search_set))
prime_list+=search_list
 
#print(prime_list)
 
 
for i in I_list:
    lower_set={j for j in prime_list if j<=i } 
#   print(lower_set)
    print(len(lower_set))

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,math.sqrt(i)):
                    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: import math


try:
    while True:
        r = int(input())

        A = [i for i in range(2, r+1)]
        P = []
        time = 0

        while True:
            prime = min(A)

            if prime > math.sqrt(r):
                break

            P.append(prime)

            i = 0
            while i < len(A):
                if A[i] % prime == 0:
                    A.pop(i)
                    continue
                i += 1
        for a in A:
            P.append(a)

        print(len(P))

except EOFError:
    pass

Python: import math

def check_prime(goal):
    true_or_false=False
    for i in range(2,int(math.sqrt(goal))+1):
        if goal%i==0:
             true_or_false=True
    if true_or_false==False:
        return True
    else:
        return False
anss=[]
while True:
    try:
        num=int(input())
        num_of_prime=0
        for i in range(2,num+1):
           if check_prime(i):
               num_of_prime+=1
        anss.append(num_of_prime)
    except:
        break

for ans in anss:
    print(ans)

Python: def main():
	import sys

	MAX = 1000000

	table = [True]*1000000
	table[0] = table[1] = False

	primes = []
	counted = 0

	for line in sys.stdin:
		n = int(line)
		if n > counted:
			for i in range(int(n+1)):
				if table[i] and not i in primes:
					primes.append(i)
					for j in range(2*i, MAX, i):
						table[j] = False
			counted = n

		cnt = 0
		for prime in primes:
			if prime > n:
				break
			else:
				cnt += 1
		print(primes)
		print(cnt)


if __name__ == '__main__':
	main()

Python: def prime_judge(n):
    if n==1:
        return False
    elif n==2:
        return True
    elif n%2==0:
        return False
    else:
        sqrt_num=int(n**0.5)+1
        for i in range(3,sqrt_num, 2):
            if n%i==0:
                return False
        return True

prime=[0]
for i in range(1,100000):
    if prime_judge(i):
        prime.append(prime[-1]+1)
    else:
        prime.append(prime[-1])

while 1:
    try:
        print(prime[int(input())])     
    except:
        break

Python: import sys
n=1000000
p=[1]*n
p[0]=p[1]=0
for i in range(n):
  if p[i]:
    for j in range(i*2,n,i):
      p[j]=0
for i in range(1,n):
  p[i]+=p[i-1]

for l in sys.stdin:
  print(p[int(l)])

Python: import sys


def primes(n):
    f = []
    for i in range(n + 1):
        if i > 2 and i % 2 == 0:
            f.append(0)
        else:
            f.append(1)
    i = 3
    while i * i <= n:
        if f[i] == 1:
            j = i * i
            while j <= n:
                f[j] = 0
                j += i + i
        i += 2
    return f[2:]


if __name__ == "__main__":
    ps = primes(100)
    for i in sys.stdin:
        print(i,sum(ps[:i-1]))

Python: import sys
lines = sys.stdin.readlines()
for line in lines:
    a = int(line)
    b = [False, False]
    for i in range(a-1):
        b.append(True)
    for i in range(a):
        if(b[i]):
            k = i
            while(k <= a-i):
                k += i
                b[k] = False
    #print(b)
    cnt = 0
    for i in b:
        if(i):
            cnt += 1
    print(cnt)