n,k=map(int,input().split())
a=list(map(int,input().split()))
mod=10**9+7
f=[1]
for i in range(n+k):f+=[f[-1]*(i+1)%mod]
def comb(a,b):return f[a]*pow(f[b],mod-2,mod)*pow(f[a-b],mod-2,mod)%mod
ans=0
for i in range(n):
  x=(i+1)
  y=(n-i)
  ans+=comb(x+k-1,x-1)*comb(y+k-1,y-1)*a[i]
  ans%=mod
print(ans)