#include "bits/stdc++.h"
using namespace std;
#define rt return
#define FOR(i,j,k) for(int i=j; i<(int)k;++i)
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define mt make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

#define N_MAX 1001
int N, K, a[N_MAX], b[N_MAX], ans = 0;
int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    cin >> N >> K;
    rep(i, N)cin >> a[i];
    rep(i, N)b[i] = a[i];
    sort(b, b + N);
    rep(i, N)a[i] = lower_bound(b, b + N, a[i]) - b;
    rep(i, N){
        if(a[i] != i){
            int ok = 0;
            for(int j = i + K; j < N; j += K){
                if(a[j] == i){
                    ok = 1;
                    for(int k = j - K; k >= i; k -= K){
                        ++ans;
                        swap(a[k], a[k + K]);
                    }
                }
            }
            if(!ok){
                cout << -1 << endl;
                rt 0;
            }
        }
    }
    cout << ans << endl;
}