#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());

    long long ans = 0;

    // Xét từng nhóm modulo k
    for (int start = 0; start < k; start++) {
        vector<long long> va, vb;
        vector<int> pos;

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

        // Kiểm tra multiset có giống nhau không
        auto sva = va, svb = vb;
        sort(sva.begin(), sva.end());
        sort(svb.begin(), svb.end());
        if (sva != svb) {
            cout << -1;
            return 0;
        }

        // Map giá trị -> vị trí trong vb
        unordered_map<long long, queue<int>> mp;
        for (int i = 0; i < (int)vb.size(); i++) {
            mp[vb[i]].push(i);
        }

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

        vector<bool> used(m, false);
        for (int i = 0; i < m; i++) {
            if (used[i]) continue;
            int cur = i, len = 0;
            while (!used[cur]) {
                used[cur] = true;
                cur = p[cur];
                len++;
            }
            if (len > 1) ans += len - 1;
        }
    }

    cout << ans;
    return 0;
}