M,N=map(int,input().split())
X=list(map(int,input().split()))
ans=0
pre=0
for now in X:
    n=now-1-pre
    ans+=n*(n+1)*(2*n+1)//6
    pre=now
n=M-pre
ans+=n*(n+1)*(2*n+1)//6
print(ans%998244353)