MOD=10**9+7

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)
      return 0 if n<k 
      return 0 if k<=0

      @fac[n] * @finv[n - k] * @finv[k] % @mod
    end
end

combi = Combi.new(200000)

n,k=gets.chomp.split.map(&:to_i)
a=gets.chomp.split.map(&:to_i).sort

ans=0

0.upto(n-1) do |i|
  ans += a[i] * combi.nCk(i,k-1)
  ans %= MOD
end

0.upto(n-1) do |i|
  ans -= a[i] * combi.nCk((n-1-i),k-1)
  ans %= MOD
end
puts ans


