#include <iostream>
#include <algorithm>
#include <cmath>
#include <vector>
#include <complex>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <iomanip>
#include <assert.h>
#include <array>
#include <cstdio>
#include <cstring>
#include <random>
#include <functional>
#include <numeric>
#include <bitset>

using namespace std;

struct before_main{before_main(){cin.tie(0); ios::sync_with_stdio(false);}} before_main;

#define REP(i,a,b) for(int i=a;i<(int)b;i++)
#define rep(i,n) REP(i,0,n)
#define all(c) (c).begin(), (c).end()
#define zero(a) memset(a, 0, sizeof a)
#define minus(a) memset(a, -1, sizeof a)
template<class T1, class T2> inline bool minimize(T1 &a, T2 b) { return b < a && (a = b, 1); }
template<class T1, class T2> inline bool maximize(T1 &a, T2 b) { return a < b && (a = b, 1); }

typedef long long ll;
int const inf = 1<<29;

int main() {
  
  int N, K; cin >> N >> K;
  vector<int> v(N);
  rep(i, N) {
    cin >> v[i];
  }

  auto u = v; sort(all(u));

  int ans = 0;

  rep(_, 1000) {
    rep(i, N-K) {
      if(v[i] > v[i+K]) { swap(v[i], v[i+K]); ans ++; }
    }
  }

  if(v == u) {
    cout << ans << endl;
  } else {
    cout << -1 << endl;
  }

  return 0;
}