mod = 998244353
inv = pow(6,mod - 2,mod)

M,N = map(int,input().split())
X = [0] + list(map(int,input().split())) + [M + 1]

ans = 0
for i in range(N + 1):
  x = X[i + 1] - X[i] - 1
  res = (x * (x + 1) % mod) * (2 * x + 1) % mod
  res = res * inv % mod
  ans = (ans + res) % mod
print(ans)