import sys

sys.setrecursionlimit(10**7)
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))
def LI2(): return list(map(int,sys.stdin.readline().rstrip()))
def S(): return sys.stdin.readline().rstrip()
def LS(): return list(sys.stdin.readline().rstrip().split())
def LS2(): return list(sys.stdin.readline().rstrip())


N,M = MI()
A = LI()
A.reverse()
mod = 10**9+7

c = 1
ans = 0
for i in range(N-1):
    a = A[i]
    c *= M+i
    c *= pow(i+1,mod-2,mod)
    c %= mod
    ans += a*c
    ans %= mod

print(ans)