MOD = 10 ** 9 + 7

n, m = map(int, input().split())
A = list(map(int, input().split()))
ans = sum(A) % MOD

times = [m - 1]
i = m
j = 2
for _ in range(n - 2):
    times.append(times[-1] * i * pow(j, MOD - 2, MOD) % MOD)
    i += 1
    j += 1

A = A[::-1]
for i in range(1, n - 1):
    times[i] += times[i - 1]

for a, t in zip(A, times):
    ans += a * t
    ans %= MOD
print(ans)