#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;
using ll = long long;

int scnt(vector<int> &vec){
    int cnt = 0;
    for (int i = 0; i < vec.size() - 1; i++){
        for (int j = 0; j < vec.size() - 1 - i; j++){
            if (vec[j] > vec[j+1]){
                swap(vec[j], vec[j+1]);
                cnt++;
            } 
        }
    }
    return cnt;
}

int main()
{
    int n, k, ans = 0;
    cin >> n >> k;
    vector<vector<int>> before(k, vector<int> (ceil((double)n/k)));
    vector<int> after(n);
    for (int i = 0; i < n; i++) cin >> before[i%k][i/k];
    for (int i = 0; i < k; i++){
        ans += scnt(before[i]);
    }
    for (int i = 0; i < n-1; i++) {
        if (before[i%k][i/k] > before[(i+1)%k][(i+1)/k]) {
            cout << -1 << endl;
            return 0;
        }
    }
    cout << ans << endl;
}