using System;
using System.Collections.Generic;
using System.Linq;
using CompLib.Util;

public class Program
{

    public void Solve()
    {
        var sc = new Scanner();
        int n = sc.NextInt();
        int[] a = sc.IntArray();

        var g = new List<Pair>();
        int target = a[0];
        int cnt = 1;
        for (int i = 1; i < n; i++)
        {
            if (target != a[i])
            {
                g.Add(new Pair(target, cnt));
                cnt = 0;
                target = a[i];
            }
            cnt++;
        }
        g.Add(new Pair(target, cnt));

        // もともと条件を満たす

        {
            var hs = new HashSet<int>();
            bool f = true;
            foreach (var p in g)
            {
                f &= hs.Add(p.A);
            }
            if (f)
            {
                Console.WriteLine(0);
                return;
            }
        }

        // 先頭と最後が同じ
        {
            var hs = new HashSet<int>();
            bool f = g[0].A == g[g.Count - 1].A;

            for (int i = 0; i < g.Count - 1; i++)
            {
                f &= hs.Add(g[i].A);
            }
            if (f)
            {
                Console.WriteLine(1);
                return;
            }
        }
        Console.WriteLine("-1");
    }

    public static void Main(string[] args) => new Program().Solve();
}

struct Pair
{
    public int A, C;
    public Pair(int a, int c)
    {
        A = a;
        C = c;
    }
}

namespace CompLib.Util
{
    using System;
    using System.Linq;

    class Scanner
    {
        private string[] _line;
        private int _index;
        private const char Separator = ' ';

        public Scanner()
        {
            _line = new string[0];
            _index = 0;
        }

        public string Next()
        {
            while (_index >= _line.Length)
            {
                _line = Console.ReadLine().Split(Separator);
                _index = 0;
            }

            return _line[_index++];
        }

        public int NextInt() => int.Parse(Next());
        public long NextLong() => long.Parse(Next());
        public double NextDouble() => double.Parse(Next());
        public decimal NextDecimal() => decimal.Parse(Next());
        public char NextChar() => Next()[0];
        public char[] NextCharArray() => Next().ToCharArray();

        public string[] Array()
        {
            _line = Console.ReadLine().Split(Separator);
            _index = _line.Length;
            return _line;
        }

        public int[] IntArray() => Array().Select(int.Parse).ToArray();
        public long[] LongArray() => Array().Select(long.Parse).ToArray();
        public double[] DoubleArray() => Array().Select(double.Parse).ToArray();
        public decimal[] DecimalArray() => Array().Select(decimal.Parse).ToArray();
    }
}