#include <bits/stdc++.h>
using namespace std;
int n, k;
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

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


    vector<long long> clone = a;
    sort(clone.begin(), clone.end());

    long long ans = 0;

    for (int r = 0; r < k; r++) {
        vector<long long> cur, target;

        for (int i = r; i < n; i += k) {
            cur.push_back(a[i]);
            target.push_back(clone[i]);
        }

        auto c1 = cur, c2 = target;
        sort(c1.begin(), c1.end());
        sort(c2.begin(), c2.end());
        if (c1 != c2) {
            cout << -1;
            return 0;
        }

        int m = cur.size();
        unordered_map<long long, queue<int>> pos;
        
        for (int i = 0; i < m; i++) {
            pos[target[i]].push(i);
        }

        vector<int> p(m);
        for (int i = 0; i < m; i++) {
            p[i] = pos[cur[i]].front();
            pos[cur[i]].pop();
        }

        vector<bool> vis(m, false);
        for (int i = 0; i < m; i++) {
            if (vis[i] || p[i] == i) continue;

            int len = 0;
            int u = i;
            
            while (!vis[u]) {
                vis[u] = true;
                u = p[u];
                len++;
            }
            ans += len - 1;
        }
    }

    cout << ans;
    return 0;
}