local mod = 1000000007
local mfl = math.floor

local function bmul(x, y)
  local x0, y0 = x % 31623, y % 31623
  local x1, y1 = mfl(x / 31623), mfl(y / 31623)
  return (x1 * y1 * 14122 + (x1 * y0 + x0 * y1) * 31623 + x0 * y0) % mod
end

local function badd(x, y)
  return (x + y) % mod
end

local function bsub(x, y)
  return x < y and x - y + mod or x - y
end

local function modpow(src, pow)
  local res = 1
  while 0 < pow do
    if pow % 2 == 1 then
      res = bmul(res, src)
      pow = pow - 1
    end
    src = bmul(src, src)
    pow = mfl(pow / 2)
  end
  return res
end

local function modinv(src)
  return modpow(src, mod - 2)
end

local function getComb(n, k)
  local ret = 1
  for i = 1, k do
    ret = bmul(ret, n + 1 - i)
    ret = bmul(ret, modinv(i))
  end
  return ret
end

local n, k = io.read("*n", "*n")
local t = {}
for i = 1, n do
  t[i] = io.read("*n")
end
table.sort(t)
local maxsum = 0

local combs = {1}
for i = 1, k - 2 do
  combs[i] = 1
end

for i = k - 1, n do
  if i == k - 1 then
    combs[i] = 1
  else
    combs[i] = bmul(combs[i - 1], i)
    combs[i] = bmul(combs[i], modinv(i - k + 1))
  end
end
-- print(table.concat(combs, " "))

for i = n, k, -1 do
  local cmb = combs[i - 1]--getComb(i - 1, k - 1)
  maxsum = badd(maxsum, bmul(cmb, t[i]))
end
local minsum = 0
for i = 1, n - k + 1 do
  local cmb = combs[n - i]--getComb(n - i, k - 1)
  minsum = badd(minsum, bmul(cmb, t[i]))
end
-- print(maxsum, minsum)
local ret = bsub(maxsum, minsum)
print(ret)
