MOD = 10**9 + 7
N, M = gets.split.map(&:to_i)
A = gets.split.map(&:to_i)
A.unshift 0

# fac = [1, 1]
# fin = [1, 1]
# inv = [0, 1]
# (2 .. N + M + 10).each do |i|
#     inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD
#     fac[i] = fac[i - 1] * i % MOD
#     fin[i] = fin[i - 1] * inv[i] % MOD
# end

# nCk = ->(n, k) {
#     return 0 if n < k or n < 0 or k < 0
#     fac[n] * (fin[k] * fin[n - k] % MOD) % MOD
# }
class Integer
  def nCr(r); x = 1; (1..r).each { |i| x = x * (self + 1 - i) * i.pow(MOD - 2, MOD) }; x; end
end

# nCk[M + N - k, N - k + 1]
x = (M + N - 1).nCr(N)
# nCk(n, k) = n! / (k! * (n - k)!)
# nCk(n - 1, k - 1) = nCk(n, k) / n / (n - k) * k
ans = (1 .. N).sum { |k| y = x * A[k - 1] % MOD; x = x * (N - k + 1) * (M + N - k).pow(MOD - 2, MOD); y } % MOD
puts ans