import java.util.*;
public class NakamaHazure {
	public static void main(String[] args) {
		check(Arrays.asList(1,1,1,1));	// => [1]
		check(Arrays.asList(1,2,1,1,1));	// => [2,1]
		check(Arrays.asList(1,2,2,1,1));	// => []
		check(Arrays.asList(1,2,2,1,3));	// => []
	}
	private static void check(List<Integer> list) {
		HashSet<Integer> set = new HashSet<Integer>();
		HashSet<Integer> minor = new HashSet<Integer>();;
		Integer major = null;
		for (Integer i : list) {
			if (!set.add(i)) {
				if (major != null && major != i) {
					System.out.println("[]");
					return;
				}
				minor.remove(i);
				major = i;
			} else {
				minor.add(i);
			}
		}
		if (set.size() == 1) {
			System.out.println("[" + set.iterator().next() + "]");
		} else if (set.size() == 2) {
			System.out.println("[" + minor.iterator().next() + "," + major + "]");
		} else {
			System.out.println("[]");
		}
	}
}