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# (.NET Core 3.1.201) (judge verdict: Time Limit Exceeded)

using System;
using System.Numerics;
using System.Linq;
using System.Collections.Generic;
using System.Text;
using System.Collections;

namespace debug
{
    class main
    {
        static void Main(string[] args)
        {
            //問題クラスを展開
            ProgramD a = new ProgramD();
            a.main();//実行する

        }

    }
    //ABC178
    class ProgramA
    {
        public void main()
        {
            //入力
            string s = Console.ReadLine();
            //30以上ならYes、それ以外はNoを出力
            if(s[s.Length  -1] == 's')
                Console.WriteLine(s + "es");
            else
                Console.WriteLine(s + "s");


        }
    }

    class ProgramB
    {
        public void main()
        {
            //入力
            int n = int.Parse(Console.ReadLine());
            int[] zorome = new int[n];
            for (int i = 0; i < n; i++)
            {
                string[] s = Console.ReadLine().Split(' ');
                if (int.Parse(s[0]) == int.Parse(s[1]))
                    zorome[i] = 1;
                else
                    zorome[i] = 0;
            }
            int count = 0;
            for(int i = 0;i < n;i++)
            {
                if (zorome[i] == 1)
                    count++;
                else
                    count = 0;

                if (count == 3)
                {
                    Console.WriteLine("Yes");
                    return;
                }

            }
            
            Console.WriteLine("No");

        }
    }

    class ProgramC
    {
        public void main()
        {

            //入力
            long n = long.Parse(Console.ReadLine());

            long count = 0;
            for(int i = 1;i < n;i++)
            {
                count += (n - 1) / i;
            }

            Console.WriteLine(count);

        }

    }

    class ProgramD
    {
        public void main()
        {
            string[] s = Console.ReadLine().Split(' ');
            long n = long.Parse(s[0]);
            long k = long.Parse(s[1]);
            long MOD = 998244353;

            long[] dp = new long[n];
            List<long> list = new List<long>();

            for(int i = 0; i < k;i++)
            {
                string[] t = Console.ReadLine().Split(' ');
                for (int j = int.Parse(t[0]); j <= int.Parse(t[1]); j++)
                    list.Add(j);
            }
            list.Sort();
            dp[0] = 1;
            int num = list.Count();
            for (int i = 0; i < n;i++)
            {
                for (int j = 0; j < num; j++)
                {
                    long temp = list[j];
                    if (i - temp >= 0)
                    {
                        dp[i] += dp[i - temp];
                        dp[i] %= MOD;
                    }
                    else
                        break;
                }
                
                
            }

            Console.WriteLine(dp[n - 1]);

        }
            
    }

    class ProgramE
    {

        public void main()
        {

        }


    }

}