import java.io.InputStream;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.InputMismatchException;
import java.util.NoSuchElementException;
import java.math.BigDecimal;
import java.math.BigInteger;
     
public class Main{
  
static PrintWriter out;
static InputReader ir;
static final int INF=Integer.MAX_VALUE;
static final long LINF=Long.MAX_VALUE;
static final int MAX=123456*2;
  
static void solve(){
 boolean[] prime=sieveOfEratosthenes(MAX);
 while(true){
  int n=ir.nextInt();
  if(n==0) return;
  int ans=0;
  for(int i=n+1;i<=2*n;i++){
   if(prime[i]) ans++;
  }
  out.println(ans);
 }
}
 
public static void main(String[] args) throws Exception{
 ir=new InputReader(System.in);
 out=new PrintWriter(System.out);
 solve();
 out.flush();
}
 
public static boolean[] sieveOfEratosthenes(int n){
 boolean[] ret=new boolean[n+1];
 Arrays.fill(ret,true);
 ret[0]=ret[1]=false;
 for(int i=2;i<=Math.sqrt(n);i++){
  if(ret[i]){
   for(int j=i+i;j<=n;j+=i){
    ret[j]=false;
   }
  }
 }
 return ret;
}
 
static class InputReader {
 private InputStream in;
 private byte[] buffer=new byte[1024];
 private int curbuf;
 private int lenbuf;
 
 public InputReader(InputStream in) {this.in=in; this.curbuf=this.lenbuf=0;}
  
 public boolean hasNextByte() {
  if(curbuf>=lenbuf){
   curbuf= 0;
   try{
    lenbuf=in.read(buffer);
   }catch(IOException e) {
    throw new InputMismatchException();
   }
   if(lenbuf<=0) return false;
  }
  return true;
 }
 
 private int readByte(){if(hasNextByte()) return buffer[curbuf++]; else return -1;}
  
 private boolean isSpaceChar(int c){return !(c>=33&&c<=126);}
  
 private void skip(){while(hasNextByte()&&isSpaceChar(buffer[curbuf])) curbuf++;}
  
 public boolean hasNext(){skip(); return hasNextByte();}
  
 public String next(){
  if(!hasNext()) throw new NoSuchElementException();
  StringBuilder sb=new StringBuilder();
  int b=readByte();
  while(!isSpaceChar(b)){
   sb.appendCodePoint(b);
   b=readByte();
  }
  return sb.toString();
 }
  
 public int nextInt() {
  if(!hasNext()) throw new NoSuchElementException();
  int c=readByte();
  while (isSpaceChar(c)) c=readByte();
  boolean minus=false;
  if (c=='-') {
   minus=true;
   c=readByte();
  }
  int res=0;
  do{
   if(c<'0'||c>'9') throw new InputMismatchException();
   res=res*10+c-'0';
   c=readByte();
  }while(!isSpaceChar(c));
  return (minus)?-res:res;
 }
  
 public long nextLong() {
  if(!hasNext()) throw new NoSuchElementException();
  int c=readByte();
  while (isSpaceChar(c)) c=readByte();
  boolean minus=false;
  if (c=='-') {
   minus=true;
   c=readByte();
  }
  long res = 0;
  do{
   if(c<'0'||c>'9') throw new InputMismatchException();
   res=res*10+c-'0';
   c=readByte();
  }while(!isSpaceChar(c));
  return (minus)?-res:res;
 }
  
 public double nextDouble(){return Double.parseDouble(next());}
  
 public BigInteger nextBigInteger(){return new BigInteger(next());}
  
 public BigDecimal nextBigDecimal(){return new BigDecimal(next());}
  
 public int[] nextIntArray(int n){
  int[] a=new int[n];
  for(int i=0;i<n;i++) a[i]=nextInt();
  return a;
 }
  
  
 public long[] nextLongArray(int n){
  long[] a=new long[n];
  for(int i=0;i<n;i++) a[i]=nextLong();
  return a;
 }
 
 public char[][] nextCharMap(int n,int m){
  char[][] map=new char[n][m];
  for(int i=0;i<n;i++) map[i]=next().toCharArray();
  return map;
 }
}
}

Measured memory_bytes: 25500