class Integer
  def mod_inverse(mod)
    self.pow(mod - 2, mod)
  end
end

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

def comb(n, k, memo)
  return memo[n][k] if memo[n][k]

  if k == 0 || n == k
    return 1
  end

  if k == 1 || k == n - 1
    return n
  end

  memo[n][k] = (comb(n - 1, k, memo) * n * (n - k).mod_inverse(MOD)) % MOD
end

memo = Hash.new { |h, k| h[k] = {} }
ans = 0

A.each.with_index(1) do |a, i|
  ans += a * comb(K + i - 1, K, memo) * comb(N - i + K, K, memo)
  ans %= MOD
end

puts ans