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