#include<bits/stdc++.h>
using namespace std;
#ifdef LOCAL_DEBUG
  #include "LOCAL_DEBUG.hpp"
#endif
#define int long long

template <class T>
struct SegmentTree{
  using F = function<T(T,T)>;
  F f;
  T inf;
  int n;
  vector<T> node;

  SegmentTree(){}
  SegmentTree(vector<T> v,T inf,F f):inf(inf),f(f){
    n = 1; while(n < v.size()) n *= 2;
    node.resize(2*n-1,inf);
    for(int i = 0; i < v.size(); i++) node[i+n-1] = v[i];
    for(int i = n-2; i >= 0; i--) node[i] = f(node[2*i+1],node[2*i+2]);
  }

  void update(int k,T val){
    k += n-1;
    node[k] = val;
    while(k > 0){
      k = (k-1) / 2;
      node[k] = f(node[2*k+1],node[2*k+2]);
    }
  }

  void add(int k,T val){
    k += n-1;
    node[k] += val;
    while(k > 0){
      k = (k-1) / 2;
      node[k] = f(node[2*k+1],node[2*k+2]);
    }
  }

  T getval(int a,int b){ return getval(a,b,0,0,n); }
  T getval(int a,int b,int k,int l,int r){
    //区間[a, b)の値を返す
    if(r <= a || b <= l) return inf;
    if(a <= l && r <= b) return node[k];
    T vl = getval(a, b, 2*k+1, l, (l+r)/2);
    T vr = getval(a, b, 2*k+2, (l+r)/2, r);
    return f(vl, vr);
  }
};

signed main(){

  int n, k; cin >> n >> k;
  vector<int> a(n);
  for(int i = 0; i < n; i++){
    cin >> a[i];
  }

  priority_queue<int,vector<int>,greater<int>> q[k]; //最小値
  for(int i = 0; i < n; i++){
    q[i%k].push(a[i]);
  }
  int pre = -1;
  for(int i = 0; i < n; i++){
    if(pre > q[i%k].top()){
      cout << -1 << endl;
      return 0;
    }
    pre = q[i%k].top();
    q[i%k].pop();
  }

  vector<vector<int>> v(k);
  map<int, int> mp;
  for(int i = 0; i < n; i++){
    v[i%k].push_back(a[i]);
    mp[a[i]]++;
  }
  int id = 0;
  for(auto &p : mp){
    p.second = id++;
  }

  int ans = 0;
  for(int i = 0; i < k; i++){
    SegmentTree<int> seg(vector<int>(n+1, 0),0,[&](int x, int y){
      return (x + y);
    });
    for(int p : v[i]){
      ans += seg.getval(mp[p]+1, n);
      seg.add(mp[p], 1);
    }
  }
  cout << ans << endl;

  return 0;
}