import java.util.Scanner;

class DLLNode<E> {
 protected E element;
 protected DLLNode<E> pred, succ;
public DLLNode(E elem, DLLNode<E>
pred, DLLNode<E> succ) {
 this.element = elem;
 this.pred = pred;
 this.succ = succ;
 }
}
class DLL<E> {
 private DLLNode<E> first, last;
 public DLL () {
 // kreiranje prazna lista
 this.first = null;
 this.last = null;
 }
 public DLLNode<E> getFirst() {
	 return first;
 }
 public DLLNode<E> getLast() {
	 return last;
 }
 public void insertFirst(E o){
	 DLLNode<E> ins = new
	DLLNode<E>(o, null, first);
	 if (first == null)
	 last = ins;
	 else
	 first.pred = ins;
	first = ins;
	}
	public void insertLast(E o){
	 if (first == null)
	 insertFirst(o);
	 else {
	 DLLNode<E> ins = new
	DLLNode<E>(o, last, null);
	 last.succ = ins;
	 last = ins;
	 }
	}
	 public E deleteFirst(){
		 if (first != null){
		 DLLNode<E> tmp = first;
		 first = first.succ;
		 first.pred = null;
		 if (first == null) last = null;
		 return tmp.element;
		 }
		 else return null;
		}
		public E deleteLast(){
		 if (first != null){
		 if (first.succ == null)
		 return deleteFirst();
		 else {
		 DLLNode<E> tmp = last;
		 last = last.pred;
		 last.succ = null;
		 return tmp.element;
		 }
		 }
		 //else throw Exception
		return null;
		}
		public void insertAfter(E o, DLLNode<E> after) {
			if(after==last){
			insertLast(o);
			return;
			}
			DLLNode<E> ins = new DLLNode<E>(o, after,
			after.succ);
			after.succ.pred = ins;
			after.succ = ins;
			}
			public void insertBefore(E o, DLLNode<E> before) {
			if(before == first){
			insertFirst(o);
			return;
			}
			DLLNode<E> ins = new DLLNode<E>(o, before.pred,
			before);
			before.pred.succ = ins;
			before.pred = ins;
			}
			public int length() {
				int ret;
				if (first != null) {
				DLLNode<E> tmp = first;
				ret = 1;
				while (tmp.succ != null) {
				tmp = tmp.succ;
				ret++;
				}
				return ret;
				} else
				return 0;
				}

}


public class PalindromeDLL {
	public static int DLLPalindrom(DLL<Integer> palindrom) {
		DLLNode<Integer> tmp1 = palindrom.getFirst();
		DLLNode<Integer> tmp2 = palindrom.getLast();
		while(tmp1!=tmp2) {
			if(tmp1.element==tmp2.element) {
				tmp1=tmp1.succ;
				tmp2=tmp2.pred;
			}
			else {
				return -1;
			}
		}
		return 1;
	}

	public static void main(String[] args) {
	Scanner in = new Scanner(System.in);
	DLL<Integer> palindrom = new DLL<Integer>();
	int n = in.nextInt();
	for(int i = 0;i<n;i++) {
		palindrom.insertLast(in.nextInt());
	}
	System.out.println(DLLPalindrom(palindrom));
	}

}