package main

import . "fmt"
import . "os"
import bf "bufio"

const MOD = 998244353

func main() {
	rd := bf.NewReader(Stdin)
	var m, n int64
	Fscan(rd,&m,&n)
	xs := make([]int64, 1, int(n)+2)
	for ; n>0 ; n-- {
		var x int64
		Fscan(rd, &x)
		xs = append(xs, x)
	}
	xs = append(xs, m+1)
	var s int64
	for i, v := range xs[1:] {
		d := v - xs[i] - 1
		a, b, c := d, d+1, 2*d+1
		switch {
			case a%2==0:
				a/=2
			case b%2==0:
				b/=2
			case c%2==0:
				c/=2
				panic("unreachable!")
		}
		switch {
			case a%3==0:
				a/=3
			case b%3==0:
				b/=3
			case c%3==0:
				c/=3
		}
		s = (s+(a%MOD)*(b%MOD)%MOD*(c%MOD)%MOD)%MOD
	}
	Println(s)
}