class BinaryIndexTree
  def initialize(size, init: 0)
    @values = Array.new(size, init)
    @size = size
  end

  # @param idx [Integer]
  # @param x [Numeric]
  def add(idx, x)
    raise 'Out of range reference' if @size <= idx

    idx += 1

    while idx <= @size
      @values[idx - 1] += x
      idx += idx & -idx
    end
  end

  def sum(l, r)
    _sum(r) - _sum(l)
  end

  private

  def _sum(idx)
    res = 0

    while idx > 0
      res += @values[idx - 1]
      idx -= idx & -idx
    end

    res
  end
end

class Array
  def inversion_number
    n = size
    bit = BinaryIndexTree.new(n + 1)
    cnt = 0

    n.times do |i|
      cnt += i - bit.sum(0, self[i])
      bit.add(self[i], 1)
    end

    cnt
  end
end

N, M = gets.split.map(&:to_i)
P = gets.split.map(&:to_i)

num = P.inversion_number

if M.even? && num.odd?
  puts -1
elsif num == 0
  puts 0
else
  if M.even?
    k = (num + M - 1) / M
    puts k * M
  else
    diff = (M - num).abs

    if num <= M
      if diff.even?
        puts M
      else
        puts 2 * M
      end
    else
      k = (diff + M - 1) / M
      n_diff = (k * M - num).abs

      if n_diff.even?
        puts k * M
      else
        puts (k + 1) * M
      end
    end
  end
end