PAT乙级1007

import java.util.Scanner;
public class Main{
    public static void main(String args[]){
        int past=2;
        int now=2;
        int num=0;
        Scanner scan =new Scanner(System.in);
        int N=scan.nextInt();
        scan.close();
        for(int i=2;i<=N;i++){
            if(isPrimer(i)){
                past=now;
                now=i;
                if(now-past==2){
                    num++;
                }
            }    
        }
        System.out.println(num);
        
    }
    public static boolean isPrimer(int i){
        for(int j=2;j<=Math.sqrt(i);j++){
            if(i%j==0){
                return false;
            }
        }
        return true;
    }
}

共勉！