import sys
sys.setrecursionlimit(10000000)
MOD = 10 ** 9 + 7
INF = 10 ** 15

def nck_nbig(n,r,mod = MOD):
    r = min(n,r)
    if r == 0:
        return 1
    num,inv_num = 1,1
    for i in range(1,r + 1):
        num = num * (n - i + 1) %mod
        inv_num = inv_num * i % mod
    return num * pow(inv_num,mod - 2,mod)%mod

def main():
	N,K = map(int,input().split())
	A = list(map(int,input().split()))
    

	ans = 0
	before_nck1 = 1
	before_nck2 = nck_nbig(N + K - 1,N - 1)
	for i in range(N):
		if i > 0:
			before_nck1 *= (K + i)
			before_nck1 *= pow(i,MOD - 2,MOD)
			before_nck1 %= MOD
			before_nck2 *= pow(K + N - i,MOD-2,MOD)
			before_nck2 *= (N - i)
			before_nck2 %= MOD
		ans += before_nck1 * before_nck2 * A[i]
		ans %= MOD
	print(ans)
if __name__ == '__main__':
	main()