import sys
input = sys.stdin.readline

N,K=map(int,input().split())
A=list(map(int,input().split()))
mod=10**9+7

Combi=dict()
Combi[K]=1
for i in range(K+1,N+K+1):
    Combi[i]=Combi[i-1]*i*pow(i-K,mod-2,mod) %mod

ANS=0
for i in range(N):
    ANS=(ANS+Combi[i+K]*Combi[N-1-i+K]*A[i])%mod

print(ANS)