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

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

def isp(n):
    a = 1
    for i in range(2, int(n ** 0.5) + 1):
        if not n % i:
            a = 0
            break
    return a

for m in ary:
    s = 0
    for i in range(2, m+1):
        if isp(i):
            s += 1
    print(s)

Python: def sieve_of_erastosthenes(num):
    input_list = [False if i % 2 == 0 or i % 3 == 0 or i % 5 == 0 else True for i in range(num)]
    input_list[0] = input_list[1] = False
    input_list[2] = input_list[3] = input_list[5] = True

    for serial in range(3, int(num ** 0.5) + 1, 2):
        
        if input_list[serial] is True:
            
            for s in range(serial ** 2, num, serial): 
                input_list[s] = False

    # print([i for i, b in enumerate(input_list) if b == True])
    return sum(input_list)


if __name__ == '__main__':
    while True:
        try:
            n = int(input())
        except:
            break

        if 5 >= n:
            print(1)
        
        else:
            print(sieve_of_erastosthenes(n))

Python: l = [True] * 100000
for i in range(2, 100000):
    if (l[i - 1]):
        for j in range(i ** 2 - 1, 100000, i):
            l[j] = False

while True:
    try:
        n = int(input())
        print(l[1:n].count(True))
    except:
        break

Python: def p(n):
    l = list(range(2,n+1))
    p = 2
    i = 0
    p = l[i]
    while True:
        pl = len(l)
        l = list(filter(lambda n: n%p != 0 or n == p,l))
        i += 1
        p = l[i]
        if pl == len(l):
            return pl

from sys import stdin
for i in stdin:
    print(p(int(i)))

Python: import sys

N = 1000
lst = [2]
for i in range(2, N)[1::2]:
    lst.append(i)
    for j in range(3, i):
        if i % j == 0:
            lst.pop(lst.index(i))
            break

for i in sys.stdin:
    for j in range(len(lst)):
        if lst[j] > int(i):
            print(j)
            break

Python: max_n = 999999
prime_list = [1] * (max_n + 1)

# 0, 1は素数ではない
prime_list[0:2] = [0, 0]

# 
for i in range(2, int(max_n**0.5) + 1):
    if prime_list[i] == 1:
        prime_list[i**2::i] = [0] * len(prime_list[i**2::i])

while True:
    try:
        n = int(input())
        print(sum(prime_list[:n + 1]))

    except EOFError:
        break

Python: import sys
import math
 
N = 1000000
primes = [1] * (N//2)
primes[0] = 0
 
for i in range(3,int(math.sqrt(N)),2):
    if primes[i//2]:
        primes[(i*i)//2::i] = [0] * len(primes[(i*i)//2::i])
 
for i in sys.stdin:
    n = int(i)
    if n < 3:
        print(n-1)
    else:
        print(sum(primes[0:(n+1)//2])+1)

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 < 3 else sum(l[0:(n+1)//6])+sum(m[0:(n-1)//6])+2
    print(r)

Python: import sys

def sieve(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

prime1m = sieve(1000000)

for line in sys.stdin:
    print(prime1m[:int(line) + 1].count(True))

Python: def Eratosthenes(n):
	N = {i:True for i in range(2,n+1)}
	primeNumbers = []
	for i in range(2,n+1):
		if N[i]:
			primeNumbers.append(i)
			j = i*2
			while j<=n:
				N.update({j:False})
				j += i
	return primeNumbers

while True:
	n = int(input())
	print(len(Eratosthenes(n)))

Python: import sys

primes = [2, 3]
for n in range(5, 999999, 2):
    isprime = True
    for i in range(len(primes)):
        if primes[i]**2 > n:
            break
        if n%primes[i] == 0:
            isprime = False
            break
    if isprime:
        primes.append(n)

numbers = []
count = 0
for line in sys.stdin:
    numbers.append(int(line))
    count += 1

counts = []
for i in range(count):
    counts.append(0)

for prime in primes:
    if prime > 999999:
        break
    for i in range(count):
        if prime <= numbers[i]:
            counts[i] += 1

for count in counts:
    print(count)

Python: def main():
    for _ in range(30):
        try:
            number = int(input())
        except EOFError:
            return None
        p = 0
        for num in range(2, number+1):
            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()

Python: # sieve of Eratosthenes
N = 1000000
isprimer = [True for i in range(N)]
isprimer[0], isprimer[1] = False, False
for i in range(2, N):
	if not isprimer[i]:
		continue
	for j in range(i * 2, N, i):
		isprimer[j] = False

while True:
	try:
		num = eval(input())
		count = 0
		for i in range(2, num + 1):
			if isprimer[i]:
				count += 1
		print(count)
	except EOFError:
		break

Python: import sys
 
 
xx = 500000
f = [0, 1] * xx
 
i = 3
while i * i <= 2 * xx:
    if f[i] == 1:
        j = i * i
        while j <= 2 * xx:
            f[j] = 0
            j += i + i
    i += 2
 
f.pop(0)
f.pop(1)
 
for n in sys.stdin:
    i = int(n)
    if i < 2:
        print(0)
    else:
        print(sum(f[:i - 1]))

Python: import sys
def isPrime(i):
    if i<2:
        return False
    else:
        for j in range(2,i):
            if j*j>n:
                break
            elif i%j==0:
                return False
        return True

def countPrime(i):
    cnt=0
    for x in range(2,i+1):
        if isPrime(x) == True:
            print(x)
            cnt+=1
    return cnt

while True:
    try:
        n=int(input())
        cnt=0
        for x in range(2,n+1):
            if isPrime(x)==True:
                cnt+=1
        print(cnt)
    except:
        break

Python: # coding=utf-8
import array


def sieve_of_eratosthenes(end: int) -> array:
    # noinspection PyUnusedLocal
    is_prime = array.array('B', (True for i in range(end+1)))
    is_prime[0] = False
    is_prime[1] = False

    primes = array.array("L")
    for i in range(2, end+1):
        if is_prime[i]:
            primes.append(i)
            for j in range(i * 2, end+1, i):
                is_prime[j] = False
    return primes


if __name__ == '__main__':
    prime_table = sieve_of_eratosthenes(1000000)
    while True:
        try:
            n = int(input())
        except EOFError:
            break
        prime_table_under_n = [x for x in prime_table if x <= n]
        print(len(prime_table_under_n))

Python: def p(n):
    l = list(range(2,n+1))
    p = 2
    i = 0
    p = l[i]
    while True:
        pl = len(l)
        l = list(filter(lambda n: n%p != 0 or n == p,l))
        i += 1
        p = l[i]
        if pl == len(l):
            break
    return len(l)

from sys import stdin
for i in stdin:
    print(p(int(i)))