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

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var a = NList;
        for (var i = 0; i < n; ++i) --a[i];
        var pos = new int[n];
        for (var i = 0; i < n; ++i) pos[a[i]] = i;
        var res = 0;
        var visited = new bool[n];
        for (var i = 0; i < n; ++i)
        {
            if (visited[i]) continue;
            var begin = a[i];
            var cur = begin;
            var count = 0;
            while (!visited[cur])
            {
                visited[cur] = true;
                cur = pos[cur];
                ++count;
            }
            res += count - 1;
        }
        WriteLine(res % 2 == 0 ? 1 : -1);
    }
}