Tell me what is wrong? task run and result is correct
package pl.codegym.task.task07.task0720;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Iterator;
/*
Przestawiono w samą porę
*/
public class Solution {
public static void main(String[] args) throws IOException {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
int N = Integer.parseInt(reader.readLine());
int M = Integer.parseInt(reader.readLine());
ArrayList<String> ar1 = new ArrayList<>();
ArrayList<String> ar2 = new ArrayList<>();
for(int i=0;i<N;i++){
ar1.add(reader.readLine());
}
Iterator<String> itr = ar1.iterator();
Iterator<String> itr2 = ar2.iterator();
for(int i=0;i<M;i++){
ar2.add(itr.next());
itr.remove();
}
ar1.addAll(ar2);
for(String str: ar1)
System.out.println(str);
}
}