mod = 10**9 + 7

N, M = map(int, input().split())
A = list(map(int, input().split()))
A.reverse()

ans = 0
cnt = M % mod
for i, a in enumerate(A, 1):
    ans += a * cnt
    ans %= mod
    cnt *= (M+i)
    cnt *= pow(i+1, mod-2, mod)
    cnt %= mod

print(ans)