m,n=map(int,input().split())
x=list(map(int,input().split()))+[m+1]
M=998244353
i6=pow(6,M-2,M)
f=lambda n:i6*n*(n+1)*(2*n+1)%M
now=0
g=0
for v in x:
  g+=f(v-1-now)
  g%=M
  now=v
print(g)