N, K = gets.split.map(&:to_i)
A = gets.split.map(&:to_i)

def inversion_count(arr)
  cnt = 0
  n = arr.size

  n.times do
    0.upto(n - 2) do |i|
      next if arr[i] <= arr[i + 1]

      cnt += 1
      arr[i], arr[i + 1] = arr[i + 1], arr[i]
    end
  end

  cnt
end

arrs = []
checked = Array.new(N, false)

N.times do |i|
  next if checked[i]

  arr = []

  i.step(N - 1, K) do |j|
    checked[j] = true
    arr << A[j]
  end

  arrs << arr
end

res = Array.new(N)
ans = 0

arrs.each_with_index do |arr, s|
  ans += inversion_count(arr)

  s.step(N - 1, K) do |i|
    res[i] = arr.shift
  end
end

if res == A.sort
  puts ans
else
  puts -1
end