using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCoder.Contest.B
{
	class Program
	{
		public static void Main(string[] args)
		{
			var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
			Console.SetOut(sw);

			Scanner cin = new Scanner();

			int n = cin.nextInt();
			var a = cin.ArrayInt(n);
			var b = new List<int>();
			var c = new List<int>();
			for (int i = 0; i < n; i++) c.Add(0);
			int now = 0;
			foreach (var e in a)
			{
				if (now != e)
				{
					b.Add(e);
					c[e - 1]++;
				}
				now = e;
			}

			int cc = c.Count(x => (x >= 2));

			// c[b.First() - 1]--;

			if (cc == 0) Console.WriteLine(0);
			else if (cc == 1 && b.First() == b.Last() && c[b.First() - 1] == 2) Console.WriteLine(1);
			else Console.WriteLine(-1);

			Console.Out.Flush();
		}
	}

	class Scanner
	{
		string[] s;
		int i;

		char[] cs = new char[] { ' ' };

		public Scanner()
		{
			s = new string[0];
			i = 0;
		}

		public string next()
		{
			if (i < s.Length) return s[i++];
			string st = Console.ReadLine();
			while (st == "") st = Console.ReadLine();
			s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
			if (s.Length == 0) return next();
			i = 0;
			return s[i++];
		}

		public int nextInt()
		{
			return int.Parse(next());
		}

		public int[] ArrayInt(int N, int add = 0)
		{
			int[] Array = new int[N];
			for (int i = 0; i < N; i++)
			{
				Array[i] = nextInt() + add;
			}
			return Array;
		}

		public long nextLong()
		{
			return long.Parse(next());
		}

		public long[] ArrayLong(int N, long add = 0)
		{
			long[] Array = new long[N];
			for (int i = 0; i < N; i++)
			{
				Array[i] = nextLong() + add;
			}
			return Array;
		}

		public double nextDouble()
		{
			return double.Parse(next());
		}

		public double[] ArrayDouble(int N, double add = 0)
		{
			double[] Array = new double[N];
			for (int i = 0; i < N; i++)
			{
				Array[i] = nextDouble() + add;
			}
			return Array;
		}
	}
}