#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> cur, need;

        for (int i = start; i < n; i += k) {
            cur.push_back(a[i]);
            need.push_back(b[i]);
        }

        // kiểm tra multiset
        vector<int> tmp1 = cur, tmp2 = need;
        sort(tmp1.begin(), tmp1.end());
        sort(tmp2.begin(), tmp2.end());

        if (tmp1 != tmp2) {
            cout << -1;
            return 0;
        }

        int sz = cur.size();

        // build permutation cur -> need
        vector<int> perm(sz, -1);
        vector<bool> used(sz, false);

        for (int i = 0; i < sz; i++) {
            for (int j = 0; j < sz; j++) {
                if (!used[j] && cur[i] == need[j]) {
                    perm[i] = j;
                    used[j] = true;
                    break;
                }
            }
        }

        // đếm cycle
        vector<bool> visited(sz, false);

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

            int cycle = 0;
            int j = i;

            while (!visited[j]) {
                visited[j] = true;
                j = perm[j];
                cycle++;
            }

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

    cout << ans;
}