require 'pp'

class Fractional
    attr_reader :mod

    def initialize(max, mod)
        @max = max
        @mod = mod
        @frac     = { 0 => 1, 1 => 1 }
        @frac_inv = { 0 => 1, 1 => 1 }
        @inv      = {         1 => 1 }

        pre_calc(max)
    end

    def pre_calc(max)
        2.upto(max) do |i|
            @frac[i] = @frac[i - 1] * i % @mod;
            @inv[i] = @mod - @inv[@mod % i] * (@mod / i) % @mod
            @frac_inv[i] = @frac_inv[i - 1] * @inv[i] % @mod
        end
    end

    def at(i)
        @frac[i]
    end

    def inv_at(i)
        @frac_inv[i]
    end
end

class Combination
    def initialize(fraction)
        @fraction = fraction
    end

    def calc(n, r)
        return 0 if n < r
        return 0 if (n < 0 || r < 0)

        denom_inv = (@fraction.inv_at(r) * @fraction.inv_at(n - r) % @fraction.mod)
        return @fraction.at(n) * denom_inv % @fraction.mod
    end
end

MOD = 10 ** 9 + 7
frac = Fractional.new(10 ** 5 + 1, MOD)
comb = Combination.new(frac)

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

j = 1
sum = 0
(n - 1).downto(k - 1) do |i|
    x = comb.calc(i, k - 1)

    sum = (sum + x * (a_n[-j] - a_n[j-1])) % MOD

    j += 1
end

puts sum
