【解決方法】この問題のアプローチは何ですか？ この問題を解決するのを手伝ってください

Two people are playing a game where Kyle gives Genny an array arr of size n. There are some positions in the array where the index (According to 1 based indexing ) is NOT prime and the element at that index is prime.

Kyle has to find those elements and give that to Genny in order of their appearance in array.

Your task is to help Genny complete his task.

NOTE :Print -1 if there are no such elements.

Input

10 
2 3 5 7 11 13 17 19 23 29

Output
2 7 13 19 23 29

私が試したこと：

import java.util.*;

public class Main {

    public static void KyleArrayGame(int n, int[] arr) {
        // write code and print here
		int index =1;
		int flag=0;
		
		for(int i=0; i<n; i++){
			if(i%2==0){
				flag=1;
				index=arr[i];
				
				System.out.print(index+" ");
			}
                 else{
                      System.out.print("-1);
		
			}
		}
    }

    public static void main(String[] args) {
        Scanner scn = new Scanner(System.in);
        int n = scn.nextInt();
        int[] arr = new int[n];
        for(int i=0;i<n;++i){
            arr[i] = scn.nextInt();
        }
        KyleArrayGame(n,arr);
    }
}