백준 4948번

난이도 실버2

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

  public static void main(String[] args) throws NumberFormatException, IOException {
    BufferedReader input = new BufferedReader(new InputStreamReader(System.in));

  
    while(true){
      int n = Integer.parseInt(input.readLine());
      int count=0;
      
      if(n==0){
        break;
      }

      int[] primenumber = new int[2*n+1];
      
      for (int i = 1; i <= 2*n; i++){
        primenumber[i] = i;
        primenumber[1] = 1;
     
    
      } 

      for(int i = 2; i<=2*n; i++){
        for(int j =2; i*j<=2*n; j++){
          primenumber[i * j] = 0;
     
        }

      }
      
      for(int i =n+1; i<=2*n; i++){
        if(primenumber[i]!=0){
        count++;
        }
        
        
      }
    System.out.println(count);
      }

  }
}