class ModCombination
  attr_reader :fac, :inv, :finv, :mod

  MOD = 10 ** 9 + 7

  def initialize(n, mod: MOD)
    @mod = mod
    @fac = [1, 1]
    @inv = [1, 1]
    @finv = [1, 1]

    (2..n).each do |i|
      @fac[i] = fac[i - 1] * i % mod
      @inv[i] = mod - inv[mod % i] * (mod / i) % mod
      @finv[i] = finv[i - 1] * inv[i] % mod
    end
  end

  def combination(n, k)
    return 0 if n < k
    return 0 if n < 0 || k < 0

    fac[n] * (finv[k] * finv[n - k] % mod) % mod
  end

  def repeated_combination(n, k)
    combination(n + k - 1, k)
  end
end

MOD = 10 ** 9 + 7
N, K = gets.split.map(&:to_i)
A = gets.split.map(&:to_i).sort

if K == 1
  puts 0
  exit
end

ans = 0
mc = ModCombination.new(N + 1)

N.times do |i|
  a = A[i]

  if i <= N - K + 1
    ans += -a * mc.combination(N - i - 1, K - 1)
  end

  if i >= N - K - 1
    ans += a * mc.combination(i, K - 1)
  end

  ans %= MOD
end

puts ans
