N, M = map(int, input().split())
A = map(int, input().split())
MOD = 10 ** 9 + 7
modinv = lambda x, mod=MOD: pow(x, mod - 2, mod)

comb = [0] * (N + 1)
comb[0] = 1
for i in range(N):
    comb[i + 1] = comb[i] * (M + i) * modinv(i + 1) % MOD

ans = 0
for i, a in enumerate(A):
    ans += a * comb[N - 1 - i]
    ans %= MOD
print(ans)