N,K=map(int,input().split())
A=["*"]+list(map(int,input().split()))

Mod=10**9+7
Inv=[0]*(N+1); Inv[1]=1
for i in range(2,N+1):
    q,r=divmod(Mod,i)
    Inv[i]=-q*Inv[r]%Mod

L=[0]*(N+1); L[1]=1
for i in range(2,N+1):
    b=(i+K-1)*Inv[i-1]%Mod
    L[i]=b*L[i-1]%Mod

R=[0]*(N+1); R[N]=1
for i in range(N-1,0,-1):
    b=(N-i+K)*Inv[N-i]
    R[i]=b*R[i+1]%Mod

X=0
for a,l,r in zip(A[1:],L[1:],R[1:]):
    X+=a*((l*r)%Mod)
    X%=Mod

print(X)