Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

Python: import sys
a=[0,0]+[1]*10**6
for i in range(999):
 if a[i]:a[i*2::i]=[0]*len(a[i*2::i])
for e in sys.stdin:print(sum(a[:int(e)+1]))

Python: while 1:
    try:
        count=0;
        x=int(input())
        if x==1:
            print(0)
        elif x==2:
            print(1)
        else:
            a=[int(i) for i in range(2,x+1)]
            for i in  range(len(a)):
                count1=0
                for j in range(2,a[i]):
                    if a[i]%j==0:
                        count1+=1
                if count1==0:
                    count+=1
            print(count)        
    except EOFError:
        break

Python: try:
    while True:
        n=int(input())
        nums = range(2, n+1)
        res = list(range(2, n+1))
        for a in nums:
            if a in res:
                tmp = []
                for b in res:
                    if b<=a or b % a != 0:
                        tmp.append(b)
                res = tmp
        print(len(res))
except:
    pass

Python: import sys
import math

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

for line in lines:
    line = int(line)
    pList = []
    sList = list(reversed(list(range(3, line + 1))[::2]))
    while sList[-1] < int(math.ceil(math.sqrt(line))) + 1 :
        pList.append(sList.pop())
        for p in pList:
            sList = list(filter(lambda i: i % p != 0, sList))
    pList.extend(sList)
    pList.append(2)
    print(len(pList))

Python: import sys
import math
M = 1000000
B = int(math.sqrt(M)+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
for i in range(5,B,6):
    p = (i-5)//6
    ini = (i*5-1)//6
    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])

for i in sys.stdin:
    n = int(i)
    r =  n-1 if n < 4 else sum(l[0:(n+1)//6])+sum(m[0:(n-1)//6])+2
    print(r)

Python: while True:
    try:
        n=int(input())
    except:
        break
    print(sum(i==2 or pow(2,i-1,i)==1 for i in range(2,n+1)))

Python: import sys
import math

N = 1000000
primes = [1] * (N//2+1)
primes[0] = 1

for i in range(3,int(math.sqrt(N))+1,2):
    if primes[i//2]:
        primes[(i*i)//2::i] = [0] * len(primes[(i*i)//2::i])
    primes[i//2] += primes[i//2-1]

for i in range(int(math.sqrt(N))+1,N+1,2):
    primes[i//2] += primes[i//2-1]


for i in sys.stdin:
    n = int(i)
    if n == 1:
        print(0)
    elif n == 2:
        print(1)
    else:
        print(primes[(n-1)//2])

Python: # coding: utf-8

import math

def sieve_of_erastosthenes(num):

    if 2 > num:
        return 0

    serial_number_list = [r for r in range(2, num + 1)]
    multiple_list = []
            
    while math.sqrt(num) >= serial_number_list[0]:
        i = 1

        serial_number = serial_number_list[0]
  
        multiple_list.append(serial_number)
                    
        while serial_number_list[len(serial_number_list) - 1] >= serial_number * i:
    
            if serial_number * i in serial_number_list:
                
                serial_number_list.pop(serial_number_list.index(serial_number * i))

            i += 1

    return len(multiple_list + serial_number_list)

if __name__ == '__main__':
    input_list = []
    
    while True:
        try:
            print(sieve_of_erastosthenes(int(input())))
        except:
            break

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

import sys
from itertools import accumulate

def input(): return sys.stdin.readline().strip()
def list2d(a, b, c): return [[c] * b for i in range(a)]
def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]
def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]
def ceil(x, y=1): return int(-(-x // y))
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]
def Yes(): print('Yes')
def No(): print('No')
def YES(): print('YES')
def NO(): print('NO')
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
MOD = 10 ** 9 + 7

def eratosthenes_sieve(n):
    """ 素数列挙(エラトステネスの篩) """

    table = [0] * (n + 1)
    prime_list = []
    for i in range(2, n + 1):
        if table[i] == 0:
            prime_list.append(i)
            for j in range(i + i, n + 1, i):
                table[j] = 1
    return prime_list

N = 1000000
primes = eratosthenes_sieve(N)
A = [0] * 1000000
for p in primes:
    A[p] = 1
acc = list(accumulate(A))

while True:
    try:
        a = INT()
        print(acc[a])
    except:
        # 入力が取れなかったら終了
        break

Python: # coding: utf-8
# Here your code !
#coding:utf-8
import math
import sys

def flatten(L):
    if isinstance(L, list):
        if L == []:
            return []
        else:
            return flatten(L[0]) + flatten(L[1:])
    else:
        return [L]

def Eratos(data):

    p_list = [] # ????????????
    tmp = []
    num = int(math.sqrt(len(data)))+1
    for i in data:
        i=data[0]
        if i>num:
            break
        else:
            p_list.append(i)
            for j in data:
                if j%i == 0:
                    data.remove(j)
        
    p_list.append(data)
    return p_list

N = input()
a = []
lists = []
for line in sys.stdin:
    # a.append(input())
    # print(line)
    x= list(range(2,(int(line)+1)))
    print(len(flatten(Eratos(x))))

# r_list = list(range(2,(x+1)))
# ans = []
# ans = Eratos(r_list)

# print(len(flatten(ans)))

Python: import sys
def main():
    for number in sys.stdin:
        p = 0
        number = int(number)
        for num in range(1, number+1,2):
            a = int(num**0.5)
            bl = True
            for k in range(2, a+1):
                if num % k == 0:
                    bl = False
                    break
            if bl:
                p += 1
        print(p)
    return None


if __name__ == '__main__':
    main()