#![allow(non_snake_case, unused_must_use, unused_imports)]
use std::io::{self, prelude::*};

fn main() {
    let (stdin, stdout) = (io::read_to_string(io::stdin()).unwrap(), io::stdout());
    let (mut stdin, mut buffer) = (stdin.split_whitespace(), io::BufWriter::new(stdout.lock()));
    macro_rules! input {
        ($t: ty, $n: expr) => {
            (0..$n).map(|_| input!($t)).collect::<Vec<_>>()
        };
        ($t: ty) => {
            stdin.next().unwrap().parse::<$t>().unwrap()
        };
    }

    type MInt = modint::ModInt<998244353>;
    let M = input!(usize);
    let N = input!(usize);
    let mut X = vec![0];

    X.append(&mut input!(usize, N));
    X.push(M + 1);
    X.dedup();

    let mut ans = MInt::default();

    let l = X.len();

    for i in 0..l - 1 {
        let d = X[i + 1] - X[i];
        ans += (2 * d - 1) * d * (d - 1) / 6;
    }

    writeln!(buffer, "{}", ans);
}

#[rustfmt::skip]
pub mod modint {use std::{fmt::*, ops::*};#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, Default)]pub struct ModInt<const P: u32>(u32);impl<const P: u32> ModInt<P> { pub fn from_raw(value: u32) -> Self { assert!(value < P); Self(value) } pub fn pow(&self, mut x: u32) -> Self { let mut a = *self; let mut r = Self::from_raw(1); while x > 0 { if x & 1 == 1 { r *= a; } a *= a; x >>= 1; } r } pub fn inv(&self) -> Self { self.pow(P - 2) }}impl<const P: u32> Add for ModInt<P> { type Output = Self; fn add(self, rhs: Self) -> Self::Output { Self((self.0 + rhs.0) % P) }}impl<const P: u32> Sub for ModInt<P> { type Output = Self; fn sub(self, rhs: Self) -> Self::Output { Self((P + self.0 - rhs.0) % P) }}impl<const P: u32> Mul for ModInt<P> { type Output = Self; fn mul(self, rhs: Self) -> Self::Output { Self(((self.0 as u64 * rhs.0 as u64) % P as u64) as u32) }}impl<const P: u32> Div for ModInt<P> { type Output = Self; fn div(self, rhs: Self) -> Self::Output { self * rhs.inv() }}impl<const P: u32> AddAssign for ModInt<P> { fn add_assign(&mut self, rhs: Self) { self.0 += rhs.0; self.0 %= P; }}impl<const P: u32> SubAssign for ModInt<P> { fn sub_assign(&mut self, rhs: Self) { self.0 += P - rhs.0; self.0 %= P; }}impl<const P: u32> MulAssign for ModInt<P> { fn mul_assign(&mut self, rhs: Self) { *self = self.clone() * rhs; }}impl<const P: u32> DivAssign for ModInt<P> { fn div_assign(&mut self, rhs: Self) { *self *= rhs.inv() }}impl<const P: u32> Neg for ModInt<P> { type Output = Self; fn neg(self) -> Self::Output { Self((P - self.0) % P) }}impl<const P: u32> Display for ModInt<P> { fn fmt(&self, f: &mut Formatter<'_>) -> Result { write!(f, "{}", self.0) }}macro_rules! impl_op_for_modint { ($($t: ty), *) => { $( impl<const P: u32> From<$t> for ModInt<P> { fn from(value: $t) -> Self { Self((P as $t + value % P as $t) as u32 % P) } } impl<const P: u32> Add<$t> for ModInt<P> { type Output = Self; fn add(self, rhs: $t) -> Self::Output { self + Self::from(rhs) } } impl<const P: u32> Add<ModInt<P>> for $t { type Output = ModInt<P>; fn add(self, rhs: ModInt<P>) -> Self::Output { Self::Output::from(self) + rhs } } impl<const P: u32> Sub<$t> for ModInt<P> { type Output = Self; fn sub(self, rhs: $t) -> Self::Output { self - Self::from(rhs) } } impl<const P: u32> Sub<ModInt<P>> for $t { type Output = ModInt<P>; fn sub(self, rhs: ModInt<P>) -> Self::Output { Self::Output::from(self) - rhs } } impl<const P: u32> Mul<$t> for ModInt<P> { type Output = Self; fn mul(self, rhs: $t) -> Self::Output { self * Self::from(rhs) } } impl<const P: u32> Mul<ModInt<P>> for $t { type Output = ModInt<P>; fn mul(self, rhs: ModInt<P>) -> Self::Output { Self::Output::from(self) * rhs } } impl<const P: u32> Div<$t> for ModInt<P> { type Output = Self; fn div(self, rhs: $t) -> Self::Output { self / Self::from(rhs) } } impl<const P: u32> Div<ModInt<P>> for $t { type Output = ModInt<P>; fn div(self, rhs: ModInt<P>) -> Self::Output { Self::Output::from(self) / rhs } } impl<const P: u32> AddAssign<$t> for ModInt<P> { fn add_assign(&mut self, rhs: $t) { *self += Self::from(rhs) } } impl<const P: u32> SubAssign<$t> for ModInt<P> { fn sub_assign(&mut self, rhs: $t) { *self -= Self::from(rhs) } } impl<const P: u32> MulAssign<$t> for ModInt<P> { fn mul_assign(&mut self, rhs: $t) { *self *= Self::from(rhs) } } impl<const P: u32> DivAssign<$t> for ModInt<P> { fn div_assign(&mut self, rhs: $t) { *self /= Self::from(rhs) } } )* };}impl_op_for_modint!(usize, isize, u64, i64, u32, i32);}