m,n=map(int,input().split())
X=[0]+list(map(int,input().split()))
mod=998244353
ans=0
X.append(m+1)
for i in range(1,n+2):
    k=X[i]-X[i-1]-1
    ans+=k*(k+1)*(2*k+1)//6
    ans%=mod
print(ans)