[Java] 베르트랑 공준

import java.util.Scanner;

public class ct2 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        while(true){
            int n = sc.nextInt();
            if(n == 0) {
                break;
            }
            boolean arr[] = new boolean[n * 2 + 1];
            arr[0] = true;
            arr[1] = true;

            for (int i = 2; i < Math.sqrt(2 * n + 1); i++){
                for (int j = i * i; j < n * 2 + 1; j++){
                    arr[j] = true;
                }
            }
            int cnt = 0;
            for (int i = n + 1; i < 2 * n + 1; i++){
                if(!arr[i]){
                    cnt++;
                }
            }
            System.out.println(cnt);
        }
    }
}