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: import math

def IsPrimer(num):
	if num == 1:
		isprimer = False
	else:
		isprimer = True
		for divisor in range(2, int(math.sqrt(num)) + 1):
			if num % divisor == 0:
				isprimer = False
				break
	return isprimer

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
a=[1]*500000
for i in range(3,999,2):
 if a[i//2]:a[i*i//2::i]=[0]*len(a[i*i//2::i])
for e in map(int,sys.stdin):print([e-1,sum(a[:(e+1)//2])][e>3])

Python: from sys import stdin
a = [True]*1000000
for i in range(2,1000000):
    if a[i]:
        for j in range(i+i, 1000000)[::i]:
            a[j] = False

b = [None]*1000000
b[0] = b[1] = 0
for i in range(2,1000000):
    if a[i]:
        b[i] = b[i-1] + 1
    else:
        b[i] = b[i-1]

while True:
    try:
        print(b[int(input())])
    except:
        break

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

        return True


while True:
    try:
        n = int(input())
    except EOFError:
        break

    count = 0
    for i in range(1, n + 1):
        count += 1 if isPrime(i) else 0

    print(count)

Python: import sys
a=[1]*500000
for i in range(3,999,2):
 if a[i//2]:a[i*i//2::i]=[0]*len(a[i*i//2::i])
for e in map(int,sys.stdin):print([e-1,sum(a[:(e+1)//2])][e>3])

Python: prime=[2]
for i in range(3,100,2):
    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: MAX = 1000000
SQRT = 1000     # sqrt(MAX)
 
prime = [0] * MAX
 
def sieve():
    for i in range(3, MAX, 2):
        prime[i] = 1
    for i in range(3, SQRT, 2):
        if prime[i] == 1:
            for j in range(i*i, MAX, i):
                prime[j] = 0
 
sieve()
cnt = [0] * (MAX+1)
cnt[2] = f = 1
for i in range(3, MAX, 2):
    if prime[i]:
        f += 1;
    cnt[i] = cnt[i+1] = f
     
while True:
    try:
        print(cnt[int(input())])
    except EOFError:
        break

Python: import sys
def fact(n):
    ret = [1]
    if n%2==0:
        ret.append(2)
        return ret
    for i in range(3,int(n**0.5)+1,2):
        if n%i==0:
            ret.append(i)
            return ret
    return ret
def isPrime(n):
    if n<=1:
        return False
    if n==2:
        return True
    if fact(n)==[1]:
        return True
    else:
        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: # -*- coding;utf-8 -*-

def sieve(n):
    p = 0
    primes = []
    is_prime = [True]*(n+1)
    is_prime[0] = False
    is_prime[1] = False
    for i in range(2, n+1):
        if(is_prime[i]):
            primes.append(i)
            p += 1
            for j in range(i*2,n,i):#iごとに増える
                is_prime[j] = False
    return p

if(__name__ == "__main__"):
    while(True):
        try:
            n = int(input())
        except:
            break
        print(sieve(n))

Python: from math import sqrt,floor
from sys import stdin

def cntPrime(n):
    mx = sqrt(n) + 1
    if n % 2 == 0:
      l = [0, 1] + [1, 0] * (int(n/2) - 1)
    else:
      l = [0, 1] + [1, 0] * (int(n/2) - 1) + [1]
    c = 3
    while c < mx:
        for k in range(c, floor(n/c) + 1, 1):
            l[c * k -1] = 0
        c = l[:c].index(1) + c
        while c % 2 == 0 or (c > 5 and c % 5 == 0): 
            c = l[:c].index(1) + c
    return sum(l)
for n in stdin:
    n = int(n)
    print(cntPrime(n))

Python: import sys
for line in sys.stdin:
    n = int(line)
    count = 0
    for i in range(2,n+1):
        k = 2
        flag = True
        while k**2 <= i:
            if i%k == 0:
                flag = False
                break
            k += 1
        if flag:
            count += 1
    print (count)

Python: import bisect
def ifprime(n):
    for i in range(3,int(n**0.5)+1):
        if n%i==0:return(False)
    return(True)
prime_list=[2,3]
while 1:
    try:
        n=int(input())
        if n>prime_list[-1]+1:
            for i in range(prime_list[-1]+2,n+1,2):
                if ifprime(i):prime_list.append(i)
        x=bisect.bisect_left(prime_list,n+1)    
        print(x)
    except:break

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

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 n in sys.stdin:
        i = int(n)
        if i < 2:
            print(0)
            break
        print(i,sum(ps[:i-1]))

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])+sum(a[1][:(e-1)//6])+2][e>3])

Python: while 1:
    try:
        b=[]
        count=1
        x=int(input())
        if x==1:
            print(0)
        elif x==2:
            print(1)
        else:
            for i in  range(3,x+1):
                count1=0
                for j in range(2,i):
                    if i%j==0:
                        count1=1
                        break
                if count1==0:
                    count+=1
            print(count)        
    except EOFError:
        break

Python: import sys
import math

N = 1000000
primes = [1] * N
primes[0] = 0
primes[1] = 0
primes[4::2] = [0] * len(primes[4::2])

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

for i in sys.stdin:
        n = int(i)
        print(sum(primes[0:n+1]))

Python: while True:
 try:
  n=int(input())
  L=[i for i in range(n+1) if i==2 or (i>=3 and i%2==1)]
  print(len(L))
 except EOFError:
  break