n,k = map(int,input().split())
A = list(map(int,input().split()))

mod = 10**9 + 7
l = 2*10**8
fac = [1]*l
facr = [1]*l

for i in range(l-1):
    fac[i+1] = fac[i]*(i+1)%mod
facr[l-1] = pow(fac[l-1],mod - 2,mod)
for i in range(1,l)[::-1]:
    facr[i-1] = facr[i]*i%mod

def combi(N,K):
    return fac[N]*facr[N-K]%mod*facr[K]%mod

ans = 0
for i,a in enumerate(A):
    ans += a*combi(i+1 +k -1,k)%mod*combi(n-i +k -1,k)%mod
print(ans)