import java.util.Scanner;

class DLLNode {
    protected E element;
    protected DLLNode pred, succ;

    public DLLNode(E elem, DLLNode pred, DLLNode succ) {
        this.element = elem;
        this.pred = pred;
        this.succ = succ;
    }

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

class DLL {
    private DLLNode 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 tmp = first;
            ret = 1;
            while (tmp.succ != null) {
                tmp = tmp.succ;
                ret++;
            }
            return ret;
        } else
            return 0;

    }

    public void insertFirst(E o) {
        DLLNode ins = new DLLNode(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 ins = new DLLNode(o, last, null);
            last.succ = ins;
            last = ins;
        }
    }

    public void insertAfter(E o, DLLNode after) {
        if (after == last) {
            insertLast(o);
            return;
        }
        DLLNode ins = new DLLNode(o, after, after.succ);
        after.succ.pred = ins;
        after.succ = ins;
    }

    public void insertBefore(E o, DLLNode before) {
        if (before == first) {
            insertFirst(o);
            return;
        }
        DLLNode ins = new DLLNode(o, before.pred, before);
        before.pred.succ = ins;
        before.pred = ins;
    }

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

    public DLLNode getFirst() {
        return first;
    }

    public DLLNode getLast() {

        return last;
    }

}

public class PalindromeDLL {

    public static int isItPalindrome(DLL list){
        DLLNode first = list.getFirst();
        DLLNode 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 list = new DLL();
        for (int i = 0; i < n; i++) {
            list.insertLast(in.nextInt());
        }
        in.close();
        System.out.println(isItPalindrome(list));
    }

}