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