#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <stack>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;

int main(){
  int N, K;
  vector<int> v;
  cin >> N >> K;
  rep(i,N){
    int a;
    cin >> a;
    v.push_back(a);
  }

  int ret = 0;
  for(int i=N-1; i>=0; i--){
    int mx = -1, mi = -1;
    rep(j,i+1){
      if(mx < v[j]){
        mx = v[j];
        mi = j;
      }
    }
    if(i == mi) continue;

    while(true){
      if(mi == i) break;
      if(mi > i){
        cout << -1 << endl;
        return 0;
      }
      if(mi+K < N){
        swap(v[mi], v[mi+K]);
        ret++;
      }
      mi += K;
    }
  }
  cout << ret << endl;
  
  return 0;
}