local ior = io.input()
local n, k = ior:read("*n", "*n")
local a = {}
for i = 1, n do a[i] = ior:read("*n") end

local cnt = 0
for i = 1, k do
  local len = math.floor((n - i) / k) + 1
  local sub = {}
  for j = 1, len do
    table.insert(sub, a[i + (j - 1) * k])
  end
  for j = len - 1, 1, -1 do
    for l = 1, j do
      if(sub[l] > sub[l + 1]) then
        cnt = cnt + 1
        sub[l], sub[l + 1] = sub[l + 1], sub[l]
      end
    end
  end
  for j = 1, len do
    a[i + (j - 1) * k] = sub[j]
  end
end
local sorted = true
for i = 2, n do
  if(a[i - 1] > a[i]) then sorted = false break end
end
if(not sorted) then print("-1") else print(cnt) end