package backjoon;

// https://www.acmicpc.net/problem/4948

// 베르트랑 공준

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStreamReader;

public class _4948 {

public static void main(String[] args) throws IOException {

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

//sol memory 24172 runtime 164

int cnt = 0;

while(true){

int N = Integer.parseInt(br.readLine());

if(N == 0) break;

boolean[] isNotPrime = new boolean[N*2+1]; //기본값이 false

isNotPrime[0] = isNotPrime[1] = true;

for (int i = 2; i <= Math.sqrt(isNotPrime.length); i++) {

if (isNotPrime[i]) {

continue;

}

for (int j = i * i; j < isNotPrime.length; j += i) {

isNotPrime[j] = true;

}

}

//N보다 크고 2N보다 작거나 같기때문에 N+1을 해준다.

for (int i = N+1; i <= N*2; i++) {

if (!isNotPrime[i]) {

cnt++;

}

}

System.out.println(cnt);

cnt = 0;

}

}

}

/*

input

1

10

13

100

1000

10000

100000

0

output

1

4

3

21

135

1033

8392

*/