#include <iostream>
#include <sstream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <functional>
#include <algorithm>

using namespace std;

#define rep(i,j) REP((i), 0, (j))
#define REP(i,j,k) for(int i=(j);(i)<(k);++i)
#define BW(a,x,b) ((a)<=(x)&&(x)<=(b))
#define ALL(v) (v).begin(), (v).end()
#define LENGTHOF(x) (sizeof(x) / sizeof(*(x)))
#define AFILL(a, b) fill((int*)a, (int*)(a + LENGTHOF(a)), b)
#define SQ(x) ((x)*(x))
#define Mod(x, mod) (((x)+(mod)%(mod))
#define MP make_pair
#define PB push_back
#define Fi first
#define Se second
#define INF (1<<29)
#define EPS 1e-10
#define MOD 1000000007

typedef pair<int, int> pi;
typedef pair<int, pi> pii;
typedef vector<int> vi;
typedef queue<int> qi;
typedef long long ll;

int N, K;
int a[1024];

int main()
{
  cin >> N >> K;
  rep(i,N) cin >> a[i];
  int ret = 0;
  for(int i=0;i<N;i++){
    for(int j=0;j+K<N;j++){
      if(a[j] > a[j+K]){
	//	cout << j << " " << j+K << endl;
	ret++;
	swap(a[j], a[j+K]);
      }
    }
  }
  for(int i=0;i<N-1;i++){
    //    cout << a[i] << " "  << a[i+1] << endl;
    if(a[i] >= a[i+1]){
      cout << -1 << endl;
      return 0;
    }
  }
  cout << ret << endl;
  return 0;
}