mod=998244353
M,N=map(int, input().split())
A=list(map(int, input().split()))
A=[0]+A
A.append(M+1)
def f(x):
  a=x*(x+1)*(2*x+1)//6
  return a 

ans=0
for i in range(len(A)-1):
  p=A[i+1]-A[i]-1
  ans+=f(p)
  ans%=mod
print(ans)