MOD = 10**9 + 7
n, k = map(int, input().split())
comb = [1, k+1]
for i in range(2, n+1):
	comb.append(comb[-1] * (k+i) * pow(i, MOD-2, MOD))

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