using System;
using System.Collections.Generic;
using System.Linq;

namespace Problem1053
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = GetInt();
            var A = GetIntArray();

            var list = new List<int>();
            int temp = 0;
            for(int i = 0; i < N; i++)
            {
                if(temp != A[i])
                {
                    list.Add(A[i]);
                    temp = A[i];
                }
            }
            var set = new HashSet<int>();
            if(list[0] == list[^1])
            {
                list.RemoveAt(list.Count - 1);
                foreach(var value in list)
                {
                    if(!set.Add(value))
                    {
                        Console.WriteLine(-1);
                        return;
                    }
                }
                Console.WriteLine(1);
            }
            else
            {
                foreach (var value in list)
                {
                    if (!set.Add(value))
                    {
                        Console.WriteLine(-1);
                        return;
                    }
                }
                Console.WriteLine(0);
            }
        }
        public static int GetInt() => int.Parse(Console.ReadLine());
        public static int[] GetIntArray() => Console.ReadLine().Split().Select(int.Parse).ToArray();
        public static double GetDouble() => double.Parse(Console.ReadLine());
        public static double[] GetDoubleArray() => Console.ReadLine().Split().Select(double.Parse).ToArray();
        public static long GetLong() => long.Parse(Console.ReadLine());
        public static long[] GetLongArray() => Console.ReadLine().Split().Select(long.Parse).ToArray();

    }
}