A[0], A[2], A[4], ... http://informatics.mccme.ru/mod/statements/view3.php?id=12660&chapterid=63#1

Main.java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int[] array = new int[in.nextInt()];
        for (int i = 0; i < array.length; i++) {
            array[i] = in.nextInt();
        }

        for (int i = 0; i < array.length; i += 2) {
            System.out.println(array[i]);
        }
    }
}