#include <bits/stdc++.h>

using namespace std;



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



int main() {
    int N,K;
    cin >> N >> K;
    assert(1 <= N && N <= 1000);
    assert(1 <= K && K <= 1000);
    vector<int> a(N);
    for(auto& it : a){
        cin >> it;
        assert(1 <= it && it <= 1000000000);
    }

    cout << babble_sort(a,K) << endl;

    return 0;
}