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

#ifdef LOCAL
#define eprintf(...) fprintf(stderr, __VA_ARGS__)
#else
#define eprintf(...) 42
#endif

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define repi(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define all(x) (x).begin(),(x).end()
#define foreach(u,v) for(auto (u) : (v))
#define pb push_back
#define mp make_pair
#define mt make_tuple

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<ll, ll> pll;
typedef vector<ll> vl;

const int inf = 1e9;
const ll linf = 1LL<<60;
const ll mod = 1e9 + 7;
const double eps = 1e-9;

/*
*/

int main()
{
  int n, k;
  cin >> n >> k;

  vi a(n);
  rep(i, n){
    cin >> a[i];
  }

  int ans = 0;
  rep(i, n){
    for(int j = n-1; j >= max(i, k); j--){
      if(a[j-k] > a[j]){
        swap(a[j-k], a[j]);
        ans++;
      }
    }
  }

  rep(i, n-1){
    if(a[i] > a[i+1]){
      cout << -1 << endl;
      return 0;
    }
  }
  cout << ans << endl;
  
  return 0;
}