using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
using System.Globalization;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    static int[] NMi => ReadLine().Split().Select(c => int.Parse(c) - 1).ToArray();
    static int[][] NMap(int n) => Enumerable.Repeat(0, n).Select(_ => NMi).ToArray();
    static string[] SList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => ReadLine()).ToArray();
    static long[] LList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => long.Parse(ReadLine())).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var a = NList;
        var count = new int[n + 1];

        var list = new List<int>();
        for (var i = 0; i < n; ++i) if (i == 0 || a[i - 1] != a[i]) list.Add(a[i]);

        foreach (var li in list) ++count[li];
        var mul = 0;
        foreach (var ci in count) if (ci > 1) ++mul;
        if (mul == 0)
        {
            WriteLine(0);
        }
        else if (mul == 1 && list[0] == list[^1])
        {
            WriteLine(1);
        }
        else
        {
            WriteLine(-1);
        }
    }
}