class Combi
  def initialize(max_n, mod = 10**9+7)
    @mod = mod
    @max_n = max_n

    @fac = Array.new(@max_n+1)
    @inv = Array.new(@max_n+1)
    @finv = Array.new(@max_n+1)

    @fac[0] = @fac[1] = 1
    @finv[0] = @finv[1] = 1
    @inv[1] = 1
    2.upto(@max_n) 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 nCk(n,k)
    @fac[n] * @finv[n - k] * @finv[k] % @mod
  end
end

n,k = gets.chomp.split().map(&:to_i)
a = gets.chomp.split().map(&:to_i)
a.sort!
a.reverse!
MOD = 10**9 + 7

combi = Combi.new(n)

maxsum = 0
minsum = 0
(n-k+1).times do |i|
  newmax = (a[i] * combi.nCk(n-i-1, k-1))%MOD
  maxsum = (maxsum + newmax) % MOD

  newmin = (a[n-i-1] * combi.nCk(n-i-1, k-1))%MOD
  minsum = (minsum + newmin) % MOD
  # p [newmax, maxsum, '', minsum, newmin]
end

ans = maxsum - minsum
ans += MOD if ans < 0
puts ans
