n,m = map(int,input().split())
A = list(map(int,input().split()))
A = A[::-1]
mod = 10**9+7

ans = 0
prod = 1
base = 1
for i in range(n-1):
    prod *= m+i
    prod %= mod
    base *= (i+1)
    base %= mod
    ans += A[i]*prod*pow(base,mod-2,mod)
    ans %= mod

print(ans)