Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	var primeCount int
	for sc.Scan() {
		input := nextInt()
		for i := 0; i <= input; i++ {
			if IsPrime(i) {
				primeCount++
			}
		}
		fmt.Printf("%d\n", primeCount)
		primeCount = 0
	}
}

func IsPrime(num int) bool {
	switch {
	case num == 2:
		return true
	case num < 2 || num%2 == 0:
		return false
	}
	sqrt := math.Sqrt(float64(num))
	for i := 3; i <= int(sqrt); i += 2 {
		if num%i == 0 {
			return false
		}
	}
	return true
}

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(n):
    if n > v:return i
  return len(L2)

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

Python: import sys,random

rand = random.randint

def prime(n):
	if n == 2: return True
	if n < 2 or n & 1 == 0: return False
	return pow(2, n-1, n) == 1

a = [prime(i) for i in range(1000000)]

for s in sys.stdin:
	i = int(s)
	print(a[:i+1].count(True))

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

def main():
    List=[]
    while True:
        try:
            IN=int(input())
            for i in range(2,IN+1):
                List.append(i)

            j=0
            while (j+1)<len(List):
                l=len(List)
                for k in range(l-1,j,-1):
                    a=List[k]%List[j]
                    if a==0:
                        del List[k]
                    else:
                        pass

                j+=1

            print(len(List))
                    
        
        except:
            break

Python: import sys
x = 1000000
xx = 500000
f = [1, 0] * xx
f[0] = 0
f[1] = 1
i = 3
while i < x:
    if f[i] == 1:
        j = i
        y = x // i + 1
        while j < y:
            f[j] = 0
            j += i + i
    i += 2

for n in sys.stdin:
    i = int(n)
    print(sum(f[:i]))

Python: def isprime(num):
   i = 2
   count = 0
   while i< num:
       if num % i == 0:
           count += 1
       i += 1
   if count == 0:
       return 1
   else:
       return 0
num = int(input())
count_prime = 0
for i in range(2,num):
   if(isprime(i) == 1):
       count_prime += 1
print(count_prime)

Python: def IsPrime(arr):
  arr[0] = 0
  arr[1] = 0
  i = 2
  while i**2 <= len(arr):
    if arr[i]:
      for j in range(i*2,len(arr),i):
        arr[j] = 0
    i += 1
  return arr

num = []
while True:
  try:
    line = int(input())
  except EOFError:
    break
  num.append(line)

n_max = max(num)
prime_array = IsPrime([1 for i in range(n_max+1)])
for i in range(len(num)):
  counter = 0
  for j in range(num[i]+1):
    counter += prime_array[j]
  print(counter)

Python: import sys

def check(q):
    if q == 2: return True
    if q&1 == 0: return False
    return pow(2, q-1, q) == 1

l = [d for d in range(2, 1000000) if check(d)]

for line in sys.stdin:
    N = int(line.rstrip())
    print(len([i for i in l if N>i]))

Python: import sys
  
lalala = 1000000//30 + 1
f = ([1,0,0,0,1,0,1,0,0,0,1,0,1,0,0,0]+[1,0]*7 ) * lalala
f = [0,0,1,1] + f[3:-20]

i = 5
while i < 1000:
    if f[i] == 1:
        j = i * i
        while j <= 1000000:
            f[j] = 0
            j += i + i
    i += 2

for n in sys.stdin:
    i = int(n)
    print(sum(f[:i + 1]))

Python: import sys
import math
M = 1000000
B = int(math.sqrt(M)+1)//6+1
N = M//6
N1 = N+1 if M % 6 == 5 else N
N2 = N+1 if M % 6 >= 1 else N
l = [1] * N1
m = [1] * N2
i = 5
ini = 4
for p in range(B):
    if l[p] == 1:
        l[p+i::i] = [0] * len(l[p+i::i])
        m[ini-1::i] = [0] * len(m[ini-1::i])
    if m[p] == 1:
        m[p+i+2::i+2] = [0] * len(m[p+i+2::i+2])
        l[ini+1::i+2] = [0] * len(l[ini+1::i+2])
    i += 6
    ini += 5
 
for i in sys.stdin:
    n = int(i)
    r =  n-1 if n < 3 else sum(l[0:(n+1)//6])+sum(m[0:(n-1)//6])+2
    print(r)

Python: import sys
results = {}
def fact(n):
    if n in results:
        return results[n]
    ret = [1]
    if n%2==0:
        ret.append(2)
        results[n]=ret
        return ret
    for i in range(3,int(n**0.5)+1,2):
        if n%i==0:
            ret.append(i)
            results[n]=ret
            return ret
    results[n]=ret
    return ret
prime_results = {}
def isPrime(n):
    if n in prime_results:
        return prime_results[n]
    if n<=1:
        return False
    if n==2:
        return True
    if fact(n)==[1]:
        prime_results[n]=True
        return True
    else:
        prime_results[n]=False
        return False
lines = sys.stdin.readlines()
for line in lines:
    inp = int(line)
    s=0
    for i in range(2,inp+1):
        if isPrime(i):
            s+=1
    print (s)

Python: from math import sqrt

I_list=[]

#input syutoku

while True:

	try:

		k=int(input())

		I_list.append(k)

	except:

		break



I_max=max(I_list)

#create list (I_max=100 -> [3,5,7,...,99])

odd_list=[x for x in range(3,I_max+1,2)]

#print(W_list)



P_list=[2]



p=0

#print(I_list)

ct=0

while p<=int(sqrt(I_max)):

	p=odd_list.pop(0)

	P_list.append(p)

	P_baisuu=[p*x for x in range(1, int(I_max/p) +1 )]

	K_list=set(odd_list)-set(P_baisuu)

	odd_list=sorted(list(K_list))

#	print("P_list=")

#	print(P_list)

#	print("odd_list=")

#	print(odd_list)



prime_list=P_list+odd_list

#print("prime_list=")

#print(prime_list)



for i in I_list:

	w=[j for j in prime_list if j<=i ]

	print(len(w))

Python: import sys

def prime(n):
    N = n + 1
    ints = [False if i % 2 == 0 or i % 3 == 0 or i % 5 == 0 else True for i in range(N)]
    try:
        ints[0] = ints[1] = False
        ints[2] = ints[3] = ints[5] = True
    except IndexError:
        pass
    sqrt = N ** 0.5

    for i in range(3, N, 2):
        if i >= sqrt:
            break
        for m in range(i ** 2, N, i):
            ints[m] = False
    return ints.count(True)

for line in sys.stdin:
    print(prime(int(line)))

Python: import sys
m=166666;a=[[1]*m for _ in[0]*2]
for i in range(m):
 if a[0][i]:
  k=6*i+5
  a[0][i+k::k]=[0]*len(a[0][i+k::k])
  a[1][-2-i+k::k]=[0]*len(a[1][-2-i+k::k])
 if a[1][i]:
  k=6*i+7
  a[0][-2-i+k::k]=[0]*len(a[0][-2-i+k::k])
  a[1][i+k::k]=[0]*len(a[1][i+k::k])
for e in map(int,sys.stdin):
 print([e-1,sum(a[0][:(e+1)//6]+a[1][:(e-1)//6])+2][e>3])