#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    int n, k;
    cin >> n >> k;
    int a[n], b[n];
    rep(i,n){
        cin >> a[i];
        b[i] = a[i];
    }
    sort(b,b+n);
    int ans = 0;
    rep(_,10000){
        rep(i,n-k){
            if(a[i] > a[i+k]){
                ans++;
                swap(a[i],a[i+k]);
            }
        }
    }

    rep(i,n)if(a[i]!=b[i])ans = -1;
    cout << ans << endl;
    return 0;
}