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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;
    }

@Override
    public String toString() {
        return "<-" + element.toString() + "->";
    }
}

class DLL<E> {
    private DLLNode<E> first, last;

public DLL() {
        // Construct an empty SLL
        this.first = null;
        this.last = null;
    }

public void deleteList() {
        first = null;
        last = null;
    }

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 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 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 E deleteFirst() {
        if (first != null) {
            DLLNode<E> tmp = first;
            first = first.succ;
            if (first != null) 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;
    }

@Override
    public String toString() {
        String ret = new String();
        if (first != null) {
            DLLNode<E> tmp = first;
            ret += tmp + "<->";
            while (tmp.succ != null) {
                tmp = tmp.succ;
                ret += tmp + "<->";
            }
        } else
            ret = "Prazna lista!!!";
        return ret;
    }

public DLLNode<E> getFirst() {
        return first;
    }

public DLLNode<E> getLast() {

return last;
    }

}

public class PalindromeDLL {

public static int isItPalindrome(DLL<Integer> list){
        DLLNode<Integer> first = list.getFirst();
        DLLNode<Integer> last = list.getLast();

while(first.element == last.element) {
            first = first.succ;
            last = last.pred;

if(first == null) {
               return 1;
            }
        }

return -1;
    }

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

}