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

mod = 10**9 + 7

ans = 0
c = 1
f = 1
for i in range(1,n+k):
    f = (f*i)%mod
    if i == k:
        c = (c*pow(f,mod-2,mod))%mod
    if i == n-1:
        c = (c*pow(f,mod-2,mod))%mod
    if i == n-1+k:
        c = c*f%mod

for i,a in enumerate(A):
    ans += a*c%mod
    ans %= mod

    c *= (k+i+1)*(n-i-1)%mod
    c %= mod
    c *= pow((i+1)*(n-1-i+k)%mod,mod-2,mod)
    c %= mod
print(ans)