PAT1007

代码如下：

import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner input = new Scanner(System.in);
        int N = input.nextInt();
        boolean flag;
        ArrayList<Integer> arr = new ArrayList<Integer>();
        for(int i = 2;i<=N;i++){
            flag = false;
            for(int j=2;j<=Math.sqrt(i);j++){
                if(i%j==0){
                    flag=true;
                    break;
                }
            }
            if(flag==false){
                arr.add(i);
            }
        }
        int count = 0;
        for(int i = 0;i<arr.size()-1;i++){
            if((int)Math.abs(arr.get(i+1)-arr.get(i))==2){
                count++;
            }
        }
        System.out.println(count);
    }
}