Problem:
Score : 400 points

Problem Statement

There are N cells arranged in a row, numbered 1, 2, \ldots, N from left to right.

Tak lives in these cells and is currently on Cell 1. He is trying to reach Cell N by using the procedure described below.

You are given an integer K that is less than or equal to 10, and K non-intersecting segments [L_1, R_1], [L_2, R_2], \ldots, [L_K, R_K].
Let S be the union of these K segments.
Here, the segment [l, r] denotes the set consisting of all integers i that satisfy l \leq i \leq r.

When you are on Cell i, pick an integer d from S and move to Cell i + d. You cannot move out of the cells.

To help Tak, find the number of ways to go to Cell N, modulo 998244353.

Constraints

2 \leq N \leq 2 \times 10^5

1 \leq K \leq \min(N, 10)

1 \leq L_i \leq R_i \leq N

[L_i, R_i] and [L_j, R_j] do not intersect (i \neq j)

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N K
L_1 R_1
L_2 R_2
:
L_K R_K

Output

Print the number of ways for Tak to go from Cell 1 to Cell N, modulo 998244353.

Sample Input 1

5 2
1 1
3 4

Sample Output 1

4

The set S is the union of the segment [1, 1] and the segment [3, 4], therefore S = \{ 1, 3, 4 \} holds.

There are 4 possible ways to get to Cell 5:

1 \to 2 \to 3 \to 4 \to 5,

1 \to 2 \to 5,

1 \to 4 \to 5 and

1 \to 5.

Sample Input 2

5 2
3 3
5 5

Sample Output 2

0

Because S = \{ 3, 5 \} holds, you cannot reach to Cell 5.
Print 0.

Sample Input 3

5 1
1 2

Sample Output 3

5

Sample Input 4

60 3
5 8
1 3
10 15

Sample Output 4

221823067

Note that you have to print the answer modulo 998244353.

Solution in Rust (1.42.0) (judge verdict: Accepted)

use std::fmt::Debug;
use std::str::FromStr;

pub struct TokenReader {
    reader: std::io::Stdin,
    tokens: Vec<String>,
    index: usize,
}

impl TokenReader {
    pub fn new() -> Self {
        Self {
            reader: std::io::stdin(),
            tokens: Vec::new(),
            index: 0,
        }
    }

    pub fn next<T>(&mut self) -> T
    where
        T: FromStr,
        T::Err: Debug,
    {
        if self.index >= self.tokens.len() {
            self.load_next_line();
        }
        self.index += 1;
        self.tokens[self.index - 1].parse().unwrap()
    }

    pub fn vector<T>(&mut self) -> Vec<T>
    where
        T: FromStr,
        T::Err: Debug,
    {
        if self.index >= self.tokens.len() {
            self.load_next_line();
        }
        self.index = self.tokens.len();
        self.tokens.iter().map(|tok| tok.parse().unwrap()).collect()
    }

    pub fn load_next_line(&mut self) {
        let mut line = String::new();
        self.reader.read_line(&mut line).unwrap();

        self.tokens = line
            .split_whitespace()
            .map(String::from)
            .collect();
        self.index = 0;
    }
}

fn solve(n: usize, inters: &Vec<(usize, usize)>) -> i64 {
    let mut dp = vec![0; n + 1];
    dp[1] = 1;

    const MOD: i64 = 998244353;
    for i in 2..=n {
        let mut curr = 0;
        for &(a, b) in inters.iter() {
            let l = if b > i { 0 } else { i - b };
            let r = if a > i { 0 } else { i - a };

            curr = (curr + dp[r]) % MOD;
            if l > 0 {
                curr = (curr + MOD - dp[l - 1]) % MOD;
            }
        }
        dp[i] = (curr + dp[i - 1]) % MOD;
    }

    (dp[n] + MOD - dp[n - 1]) % MOD
}

fn main() {
    let mut reader = TokenReader::new();
    let n = reader.next();
    let k = reader.next();
    let inters = (0..k).map(|_| (reader.next(), reader.next())).collect();

    let res = solve(n, &inters);
    println!("{}", res);
}