mod = 10**9+7


def inv(a, mod):
    r = [1, 0, a]
    w = [0, 1, mod]
    while w[2] != 1:
        q = r[2]//w[2]
        r_new = [r[0]-q*w[0], r[1]-q*w[1], r[2]-q*w[2]]
        r = w
        w = r_new
    x, y = w[0], w[1]    # a*x+y*mod = 1
    return (mod+x % mod) % mod


max_num = 2*10**5+1  # 10**6 にしたほうがよい?
fact = [0 for _ in range(max_num)]
ifact = [0 for _ in range(max_num)]

fact[0] = fact[1] = 1
ifact[0] = ifact[1] = 1

for i in range(2, max_num):
    fact[i] = fact[i-1] * i % mod

ifact[max_num-1] = inv(fact[max_num-1], mod)

for i in range(2, max_num):
    ifact[max_num-i] = (ifact[max_num-i+1] * (max_num-i+1)) % mod


def comb(x, y):
    if x < 0 or y < 0 or x < y:
        return 0
    else:
        return fact[x] * ifact[y] * ifact[x-y] % mod


n, k = map(int, input().split())
a = [int(x) for x in input().split()]

ans = 0
cl = 1
cr = ifact[n-1]
for i in range(n-1):
    cr *= (k+n-1-i)
    cr %= mod


for i in range(n):
    ans += a[i]*cl*cr
    ans %= mod
    if i == n-1:
        continue
    cl *= (k+i+1)*inv(i+1, mod)
    cr *= (n-1-i)*inv(k+n-1-i, mod)
    cl %= mod
    cr %= mod

print(ans)