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 C++ (GCC 9.2.1) (judge verdict: Time Limit Exceeded)

/* [Template].cpp 
Example code to use when starting new code. This particular problem was problem 
977A_Wrong_Subtraction from CodeForces

Compile: g++  -o D.exe D.cpp 
Execute: ./D
*/ 

#include <cstdio>
#include <iostream>
#include <cmath> 
#include <limits> 
#include <iomanip>
#include <cstring> 
#include <bits/stdc++.h>
#include <string>  
using namespace std;

int main() { 
	// Input 
	int n, k;
	cin >> n >> k;
	int left[k], right[k]; 
	for(int i = 0; i < k; i++) {
		cin >> left[i] >> right[i]; 
	} 
	int mod = 998244353; 
	
	// Solve -- naive? O(N^2) worst case. 
	int r[n+1] = {0}; 
	r[1] = 1; 
	for(int i = 1; i < n; i++) { 
		for(int w = 0; w < k; w++) {
			for(int j = left[w]; j <= right[w] && i+j<=n; j++) {
				// cout << i << "\t" << j << "\t" << r[i+j] << endl; 
				r[i+j] += r[i]; 
				r[i+j] %= mod; 
			}
		}
	}
	
	cout << r[n] << endl; 
	
	return 0; 
}