#include <algorithm>
#include <numeric>
#include <string>
#include <vector>
#include <iostream>
#include <queue>
#include <utility>
#include <cmath>
#include <bitset>

using namespace std;

bool check(int index1,int index2,int k)
{
    int sub = abs(index1 - index2);
    if(sub % k == 0) return true;
    else return false;
}

int main()
{
    int n,k;
    cin >> n >> k;
    vector<int> a(n);

    for(auto&& x : a)
    {
        cin >> x;
    }

    vector<int> sorted_a = a;
    sort(sorted_a.begin(),sorted_a.end());

    if(k > n)
    {
        if(sorted_a == a)
        {
            cout << 0 << endl;
            return 0;
        }
        else
        {
            cout << -1 << endl;
            return 0;
        }
    }

    int count = 0;
    int i = 0;

    for(int i =0; i < n; ++i)
    {
        auto iter1 = find(a.begin(), a.end(),a.at(i));
        int index1 = distance(a.begin(), iter1);
        auto iter2 = find(sorted_a.begin(), sorted_a.end(),a.at(i));
        int index2 = distance(sorted_a.begin(), iter2);

        if(!check(index1,index2,k))
        {
            cout << -1 << endl;
            return 0;
        }
    }

    while(a != sorted_a)
    {
        if(sorted_a.at(i) != a.at(i))
        {
            if(i+k < n)
            {
                swap(a.at(i),a.at(i+k));
                ++count;
            }
        }
        ++i;

        if(i == n)
            i = 0;
    }

    cout << count << endl;



}