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

class Iroha
{
    public Iroha() { }
    public static int Main()
    {
        new Iroha().calc();
        return 0;
    }

    Scanner cin;
    
    void calc()
    {
        cin = new Scanner();
        int N = cin.nextInt();
        int K = cin.nextInt();
        int[] a = new int[N];
        int[] ans = new int[N];
        int ret = 0;

        for (int i = 0; i < N; i++)
        {
            a[i] = cin.nextInt();
        }

        for (int i = 0; i < K; i++)
        {
            List<int> num = new List<int>();
            for (int j = i; j < N; j += K)
            {
                num.Add(a[j]);
            }

            for (int j = 0; j < num.Count; j++)
            {
                for (int k = 0; k < num.Count - 1; k++)
                {
                    if(num[k] > num[k + 1])
                    {
                        int c = num[k];
                        num[k] = num[k + 1];
                        num[k + 1] = c;
                        ret++;
                    }
                }
            }

            for (int j = 0; j < num.Count; j++)
            {
                ans[i + j * K] = num[j];
            }
        }

        for (int i = 0; i < N - 1; i++)
        {
            if(ans[i] > ans[i + 1])
            {
                Console.WriteLine(-1);
                return;
            }
        }
        Console.WriteLine(ret);

    }
}

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);
        i = 0;
        return next();
    }

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

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

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

}