n,k=gets.split.map(&:to_i)

aa=gets.split.map(&:to_i)

@cnt=0
def solve_sort!(arr,right,hop)
  i=right
  j=right-hop
  while (0<=j)
    if arr[j]>arr[i]
      arr[i],arr[j]=arr[j],arr[i]
      @cnt+=1
    end
    i-=hop
    j-=hop
  end
end

def sorted?(aa)
  aa.inject{|s,e|!s&&s>e ? false : e}
end

k.times{|i|
  solve_sort!(aa,aa.size-1-i,k)
}

p sorted?(aa) ? @cnt : -1