#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define db double
#define sti string
#define vt vector
#define INF ((int) 1e9)
#define MOD 1000000007
#define BASE 311
#define pii pair<int, int>
#define pil pair<int, ll>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define pdd pair<db, db>
#define all(x) x.begin(), x.end()
#define bit(mask, i) ((mask >> i) & 1)
#define fi first
#define sc second

const int maxn = 305;
int a[maxn];
int n,k;

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin >> n >> k;
    for(int i = 0;i < n;i++){
        cin >> a[i];
    }
    int count = 0;
    for(int j =0;j < n;j++){
        for(int i =0;i < n - k;i++){
            if(a[i] > a[i + k]){
                swap(a[i], a[i+k]);
                count++;
            }
        }
    }
    for(int i = 0; i < n - 1;i++){
        if(a[i] > a[i+1]){ 
            cout << -1;
            return 0;
        }
    }
    cout << count;
}