local band = bit.band
local FenwickTree = {}
FenwickTree.create = function(self, n)
  self.n = n
  self.v = {}
  for i = 1, n do self.v[i] = 0 end
end
FenwickTree.add = function(self, pos, val)
  while pos <= self.n do
    self.v[pos] = self.v[pos] + val
    pos = pos + band(pos, -pos)
  end
end
FenwickTree.sum = function(self, r)
  local ret = 0
  while 0 < r do
    ret = ret + self.v[r]
    r = r - band(r, -r)
  end
  return ret
end
FenwickTree.new = function(n)
  local obj = {}
  setmetatable(obj, {__index = FenwickTree})
  obj:create(n)
  return obj
end

local ffi = require("ffi")
local C = ffi.C
ffi.cdef[[
long long atoll(const char*);
]]

local function lltonumber(str)
  return C.atoll(str)
end

local n, m = io.read():match("(%d+) (%d+)")
n = tonumber(n)
m = lltonumber(m)
local v = 0
local fw = FenwickTree.new(n)
for i = 1, n do
  local p = io.read("*n")
  v = v + i - 1 - fw:sum(p)
  fw:add(p, 1)
end
local a = 0LL
while a < 1LL * v do
  a = a + m
end
if (a - 1LL * v) % 2LL == 0LL then
  local ans = tostring(a):gsub("LL", "")
  print(ans)
else
  if v % 2 == 1 then
    print(-1)
  else
    local ans = tostring(a + m):gsub("LL", "")
    print(ans)
  end
end