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

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

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

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

    unordered_map<long long, vector<int>> pos;
    for(int i = 0; i < n; i++)
        pos[b[i]].push_back(i);

    unordered_map<long long, int> idx;
    vector<int> to(n);

    for(int i = 0; i < n; i++) {
        int p = pos[a[i]][idx[a[i]]++];
        if(p % k != i % k) {
            cout << -1;
            return 0;
        }
        to[i] = p;
    }

    vector<bool> vis(n, false);
    int ans = 0;

    for(int i = 0; i < n; i++) {
        if(vis[i] || to[i] == i) continue;

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

    cout << ans;
}