题解 | #双素数#

import java.util.Scanner;

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

        while(in.hasNext()){
            solution(in);
        }
    }

    /**
     * 模拟法
     * @param in
     */
    private static void solution(Scanner in){
        int k = in.nextInt();

        StringBuilder sb;
        int positive,reverse;
        for(int i=2,count=0; count<k; i++){
            sb = new StringBuilder(String.valueOf(i));
            positive = i;
            reverse = Integer.parseInt(String.valueOf(sb.reverse()));
            if(positive != reverse){
                // 双素数
                if(isPrime(positive) && isPrime(reverse)){
                    count++;
                    if(count == k){
                        if(positive <= 1000000){
                            System.out.println(positive);
                        }else{
                            System.out.println(-1);
                        }
                        break;
                    }
                }
            }
        }
    }

    /**
     * 是否素数
     * @param num
     * @return
     */
    private static boolean isPrime(int num){
        for(int i=2; i<=Math.sqrt(num); i++){
            if(num%i == 0){
                return false;
            }
        }

        return true;
    }
}