#solve atcoder problem
#exec({'RUBY_THREAD_VM_STACK_SIZE'=>'100000000'},'/usr/bin/ruby', $0) if !ENV['RUBY_THREAD_VM_STACK_SIZE']
#memo: (1..n).each do |d|
require 'prime'
require 'matrix'

def k2(k)
  return 1 if(k == 0)
  return $n if(k == 1)
  ans = 1
  if((k%2) == 0)
    ans *= k2(k/2)**2
  else
    ans *= k2(k-1)
    ans *= $n
  end
  return ans % $m
end

def k2b(n,k,m=1_000_000_007) $n=n;$m=m; return k2(k) end

def inpf() a=gets.chomp.split(" ").map(&:to_f)end

def inps() a=gets.chomp.split(" ")end

def copy(a) Marshal.load(Marshal.dump(a)) end

def kaijo(n,r = 10**9+7)(n < 2)? 1 : (2..n).inject{|memo,u|memo=(memo*u)%r} end

def na(n,d=0) Array.new(n,d)end

def na2(n,m,d=0) Array.new(n){Array.new(m,d)}end

def na3(n,m,l,d=0) Array.new(n){Array.new(m){Array.new(l,d)}}end

def inp() a=gets.chomp.split(" ").map(&:to_i)end

def r_up(a, b) (a+b-1)/b end

m,n = inp
x = inp + [m+1]
ans = 0
now = 0
x.each do |a|
  d = a-now-1
  ans += (d+1)*d*(2*d+1)/6
  now = a
  ans %= 998244353
end
puts ans

=begin
7 0
3 7 5 2 10 3 20

=end