MOD = 10**9 + 7
list_size = 100300000

f_list = [1] * list_size
f_r_list = [1] * list_size

for i in range(list_size-1):
	f_list[i+1] = (f_list[i] * (i+1)) % MOD

f_r_list[-1] = pow(f_list[-1], MOD - 2, MOD)

for i in range(list_size-2, -1, -1):
	f_r_list[i] = (f_r_list[i+1] * (i+1)) % MOD

def comb(n, r):
	if n < r or r < 0:
		return 0
	elif n == 0 or r == 0 or n == r:
		return 1
	else:
		return (f_list[n] * f_r_list[n-r] * f_r_list[r]) % MOD 

n, k = map(int, input().split())
a = list(map(int, input().split()))
ans = 0
for i, x in enumerate(a):
	j = n-i-1
	ans += comb(k+i, i) * comb(k+j, j) * x
	ans %= MOD
print(ans)