#include <iostream>
#include <utility>
#include <algorithm>
using namespace std;

int main(){
    int n, k, a[1000], aa[1000], ans=0;
    cin >> n >> k;
    for(int i=0; i<n; ++i){
        cin >> a[i];
        aa[i] = a[i];
    }
    for(int i=0; i<k; ++i){
        int nn = (n - i - 1) / k;
        for(int i2=0; i2<nn; ++i2){
            for(int i3=0; i3<nn; ++i3){
                if(a[i3*k+i] > a[(i3+1)*k+i]){
                    ++ans;
                    swap(a[i3*k+i], a[(i3+1)*k+i]);
                }
            }
        }
    }

    sort(aa, aa+n);
    for(int i=0; i<n; ++i){
        if(a[i] != aa[i]){
            cout << -1 << endl;
            return 0;
        }
    }
    cout << ans << endl;
}