#include <bits/stdc++.h>
using namespace std;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, k;
    cin >> n >> k;

    vector<int> a(n);
    for(int i = 0; i < n; i++)
        cin >> a[i];

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

    long long ans = 0;

    for(int start = 0; start < k; start++){

        vector<int> group;
        for(int i = start; i < n; i += k)
            group.push_back(a[i]);

        vector<int> sorted_group = group;
        sort(sorted_group.begin(), sorted_group.end());

        // check khả thi
        int idx = 0;
        for(int i = start; i < n; i += k){
            if(sorted_group[idx++] != b[i]){
                cout << -1;
                return 0;
            }
        }

        int sz = group.size();

        vector<pair<int,int>> arr(sz);
        for(int i = 0; i < sz; i++)
            arr[i] = {group[i], i};

        sort(arr.begin(), arr.end());

        vector<bool> visited(sz, false);

        for(int i = 0; i < sz; i++){
            if(visited[i] || arr[i].second == i)
                continue;

            int cycle = 0;
            int j = i;

            while(!visited[j]){
                visited[j] = true;
                j = arr[j].second;
                cycle++;
            }

            if(cycle > 1)
                ans += cycle - 1;
        }
    }

    cout << ans;
}