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: Time Limit Exceeded)

use proconio::{input, fastout};

#[fastout]
fn main() {
    const MODNUM: i64 = 998244353;
    input!{
        n: usize,
        k: usize,
        lr:[(i64, i64); k]
    }
    let mut flg = vec![false; n + 1];
    for &(l, r) in &lr {
        for i in l..=r {
            flg[i as usize] = true;
        }
    }
    // d[n] = 0 ~ n までいく通り数
    // ans = d[n-1]
    // d[i] = sigma[d[k]] (0 ~ i-1 if k in lr)
    let mut d = vec![0; n+1];
    d[0] = 0;
    for i in 0..=n {
        if flg[i] {
            d[i] = 1;
        }
    }
    for i in 0..n {
        for dlt in 1..i {
            if flg[dlt] {
                d[i] += d[i - dlt];
            }
        }
    }

    let ans = d[n-1] % MODNUM;
    println!("{}", ans);
}