N, M = map(int, input().split())
N -= 1
A = list(map(int, input().split())) + [0]
mod = 10**9 + 7
comb = M + 1
ans = 0
now = 1
for i in range(N - 1, -1, -1):
    ans += (A[i] - A[i - 1]) * (comb - 1)
    ans %= mod
    comb *= (M + 1 + now) * pow(now + 1, mod - 2, mod)
    comb %= mod
    now += 1
    
print(ans)