local mod = 1000000007
local mfl = math.floor

local function bmul(x, y)
  local x1, y1 = mfl(x / 31623), mfl(y / 31623)
  local x0, y0 = x - x1 * 31623, y - y1 * 31623
  return (x1 * y1 * 14122 + (x1 * y0 + x0 * y1) * 31623 + x0 * y0) % mod
end

local function badd(x, y)
  return (x + y) % mod
end

local function bsub(x, y)
  return x < y and x - y + mod or x - y
end

local function modpow(src, pow)
  local res = 1
  while 0 < pow do
    if pow % 2 == 1 then
      res = bmul(res, src)
      pow = pow - 1
    end
    src = bmul(src, src)
    pow = mfl(pow / 2)
  end
  return res
end

local function modinv(src)
  return modpow(src, mod - 2)
end

local fact = {1}
local invs = {1}
local invfact = {1}
for i = 2, 200000 do
  fact[i] = bmul(fact[i - 1], i)
  invs[i] = bmul(mfl(mod / i), mod - invs[mod % i])
  invfact[i] = bmul(invfact[i - 1], invs[i])
end

local ffi = require("ffi")
local C = ffi.C
ffi.cdef[[
long long atoll(const char*);
]]

local function lltonumber(str)
  return C.atoll(str)
end

local n, m = io.read():match("(%d+) (%d+)")
n = tonumber(n)
m = lltonumber(m)
local a = {}
for i = 1, n - 1 do
  a[i] = io.read("*n")
end

m = m % 1000000007LL
m = tostring(m):gsub("LL", "")
m = tonumber(m)
local comb = 1
local ret = 0
for i = 1, n - 1 do
  local ai = a[n - i]
  comb = bmul(comb, m)
  comb = bmul(comb, invs[i])
  ret = badd(ret, bmul(comb, ai))
  m = (m + 1) % mod
end
print(ret)