def square_sum(x):
	return x*(x+1)*(2*x+1)//6
M,N=map(int,input().split())
if N==0:
	print(square_sum(M)%998244353)
	exit()
X=[0]+list(map(int,input().split()))+[M+1]
ans=0
for i in range(1,len(X)):
	seg=X[i]-X[i-1]-1
	ans+=square_sum(seg)
	ans%=998244353
print(ans)