#define _CRT_SECURE_NO_WARNINGS
// #define _GLIBCXX_DEBUG
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <deque>
#include <complex>
#include <stack>
#include <queue>
#include <cstdio>
#include <cctype>
#include <cstring>
#include <ctime>
#include <iterator>
#include <bitset>
#include <numeric>
#include <list>
#include <iomanip>
#include <cassert>
#include <array>
#include <tuple>
#include <initializer_list>
#include <unordered_set>
#include <unordered_map>
#include <forward_list>
using namespace std;
typedef long long ll;
#define rep(i,n) for (int i = 0; i < (int)(n); i++)
#define all(c) begin(c), end(c)

using vi = vector<int>;
using vii = vector < vector<int>>;

int solve(vector<int> &a) {
    int res = 0;
    int n = a.size();
    for (int i = 0; i < n; ++i) {
        for (int j = i + 1; j < n; ++j) {
            if (a[i] > a[j]) {
                swap(a[i], a[j]);
                ++res;
            }
        }
    }
    return res;
}

int main() {
    int n;
    int l;
    while (cin >> n >> l) {
        vector<int> a(n);
        for (size_t i = 0; i < n; i++) {
            cin >> a[i];
            --a[i];
        }
        vector<int> b(a.begin(), a.end());
        sort(b.begin(), b.end());
        for (size_t i = 0; i < n; i++) {
            a[i] = lower_bound(b.begin(), b.end(), a[i]) - b.begin();
        }

        bool ok = true;
        map<int, vector<int>> m;
        for (size_t i = 0; i < n; i++) {
            if (a[i] % l != i%l) {
                ok = false;
            } else {
                m[a[i] % l].push_back(a[i]);
            }
        }
        if (!ok) {
            cout << -1 << endl;
            continue;
        }
        int ans = 0;
        for (auto &e : m) {
            ans += solve(e.second);
        }
        cout << ans << endl;
    }
}