m,n=map(int,input().split())
a = list(map(int, input().split()))

score = 0
now = 1
smallMOD = 998244353
for i in range(n):
  k = a[i] - now
  score += k * (k + 1) * (2 * k + 1) // 6
  score %= smallMOD
  now = a[i] + 1
k = m + 1 - now
score += k * (k + 1) * (2 * k + 1) // 6
score %= smallMOD
print(score)