N, K = map(int, input().split())
MOD = 10**9 + 7
A = list(map(int, input().split()))
S = [0]
for a in A:
    S.append((S[-1] + a) % MOD)


B = [0] * (N+1)
B[1] = 1
b1 = b2 = 1
for i in range(2, N+1):
    b1 *= K+i-1
    b1 %= MOD
    b2 *= i-1
    b2 %= MOD
    B[i] = b1 * pow(b2, MOD-2, MOD) % MOD

C = [(B[i]-B[i-1])%MOD for i in range(N+1)]
SC = [0]
for c in C[1:]:
    SC.append((SC[-1] + c) % MOD)

SpC = [0] * (N+1)
for i in range(1, N+1):
    SpC[i] = S[i] * C[N-i+1] % MOD

SSpC = [0] * (N+1)
SSpC[N] = SpC[N]
for i in range(N-1, 0, -1):
    SSpC[i] = (SpC[i] + SSpC[i+1]) % MOD


ans = 0
for i in range(1, N+1):
    ans += C[i] * SSpC[i] % MOD
    ans -= S[i-1] * C[i] * SC[N-i+1] % MOD
    ans %= MOD

print(ans)