# import pypyjit
# pypyjit.set_param('max_unroll_recursion=-1')
from collections import defaultdict as dd
S = input
R = range
P = print
def I(): return int(S())
def M(): return map(int, S().split())
def L(): return list(M())
def O(): return list(map(int, open(0).read().split()))
def yn(b): print("Yes" if b else "No")
biga = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
smaa = "abcdefghijklmnopqrstuvwxyz"
ctoi = lambda c: ord(c) - ord('a')
itoc = lambda i: chr(ord('a') + i)
inf = 10 ** 18
mod = 998244353
def acc(a):
	b = [0]
	for i in a:
		b.append(b[-1] + i)
	return b

m,n = M();x = L()
x=[0]+x+[m+1]
ans = 0
for i in R(n+1):
	d=x[i+1]-x[i]-1
	ans+=d*(d+1)*(2*d+1)//6
P(ans%998244353)