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: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Threading;
using System.Runtime.CompilerServices;
using System.Text;
using System.Diagnostics;
using System.Numerics;
using static System.Console;
using static System.Convert;
using static System.Math;
using static Template;
using SC = Scanner;

public partial class Solver
{
    public void Solve()
    {
        int N, K;
        sc.Make(out N, out K);
        int[] L = new int[K], R = new int[K];
        for (int i = 0; i < K; i++)
        {
            sc.Make(out L[i], out R[i]);
        }
        var dp = new SegmentTree<ModInt>(N + 1, 0, (a, b) => a + b, (a, b) => a + b);
        dp.Update(1, 1);
        for(int i = 2; i <= N; i++)
        {
            for(int j = 0; j < K; j++)
            {
                int l = Max(0, i - R[j]);
                var r = Max(0, i - L[j] + 1);
                dp.Update(i, dp.Query(l, r));
            }
        }
        Console.WriteLine(dp[N]);
    }
}

public class SegmentTree<T>
{
    protected readonly T[] dat;
    protected readonly int _N, sz;
    protected readonly Func<T, T, T> merge;
    protected readonly Func<T, T, T> update;
    protected readonly T id;
    private bool finished = true;
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    protected int Left(int i)
        => i << 1;
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    protected int Right(int i)
        => (i << 1) | 1;
    public T this[int i]
    {
        get { return dat[i + sz]; }
        set { finished = false; dat[i + sz] = value; }
    }

    public SegmentTree(int N, T id, Func<T, T, T> merge, Func<T, T, T> update = null)
    {
        _N = N;
        this.sz = 1;
        while (sz < N) sz <<= 1;
        this.id = id;
        this.merge = merge;
        this.update = update ?? ((T val1, T val2) => val2);
        dat = Create(sz << 1, () => id);
    }

    public void Update(int i, T value)
    {
        i += sz;
        dat[i] = update(dat[i], value);
        while (i > 1)
        {
            i >>= 1;
            dat[i] = merge(dat[Left(i)], dat[Right(i)]);
        }
    }

    public void Build()
    {
        for (int i = sz - 1; i > 0; i--)
            dat[i] = merge(dat[Left(i)], dat[Right(i)]);
        finished = true;
    }

    public virtual T Query(int left, int right)
    {
        if (!finished) throw new Exception("You need to Execute \"Build()\"");
        T l = id, r = id;
        for (left += sz, right += sz; left < right; left >>= 1, right >>= 1)
        {
            if ((left & 1) == 1) l = merge(l, dat[left++]);
            if ((right & 1) == 1) r = merge(dat[--right], r);
        }
        return merge(l, r);
    }
    public int MaxRight(int l, Func<T, bool> f)
    {
        if (l == _N) return _N;
        l += sz;
        var sm = id;
        do
        {
            while ((l & 1) == 0) l >>= 1;
            if (!f(merge(sm, dat[l])))
            {
                while (l < sz)
                {
                    l <<= 1;
                    if (f(merge(sm, dat[l])))
                    {
                        sm = merge(sm, dat[l]);
                        l++;
                    }
                }
                return l - sz;
            }
            sm = merge(sm, dat[l]);
            l++;
        } while ((l & -l) != l);
        return _N;
    }
    public int MinLeft(int r, Func<T, bool> f)
    {
        if (r == 0) return 0;
        r += sz;
        var sm = id;
        do
        {
            r--;
            while (r > 1 && (r & 1) == 1) r >>= 1;
            if (!f(merge(dat[r], sm)))
            {
                while (r < sz)
                {
                    r = (r << 1) | 1;
                    if (f(merge(dat[r], sm)))
                    {
                        sm = merge(dat[r], sm); r--;
                    }
                }
                return r + 1 - sz;
            }
            sm = merge(dat[r], sm);
        } while ((r & -r) != r);
        return 0;
    }
}
public struct ModInt
{
    //public const long MOD = (int)1e9 + 7;
    public const long MOD = 998244353;
    public long Value { get; set; }
    public ModInt(long n = 0) { Value = n; }
    private static ModInt[] fac, inv, facinv;
    public override string ToString() => Value.ToString();
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt operator +(ModInt l, ModInt r)
    {
        l.Value += r.Value;
        if (l.Value >= MOD) l.Value -= MOD;
        return l;
    }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt operator -(ModInt l, ModInt r)
    {
        l.Value -= r.Value;
        if (l.Value < 0) l.Value += MOD;
        return l;
    }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt operator *(ModInt l, ModInt r) => new ModInt(l.Value * r.Value % MOD);
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt operator /(ModInt l, ModInt r) => l * Pow(r, MOD - 2);
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static implicit operator long(ModInt l) => l.Value;
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static implicit operator ModInt(long n)
    {
        n %= MOD; if (n < 0) n += MOD;
        return new ModInt(n);
    }

    public static ModInt Pow(ModInt m, long n)
    {
        if (n == 0) return 1;
        if (n % 2 == 0) return Pow(m * m, n >> 1);
        else return Pow(m * m, n >> 1) * m;
    }

    public static void Build(int n)
    {
        fac = new ModInt[n + 1];
        facinv = new ModInt[n + 1];
        inv = new ModInt[n + 1];
        inv[1] = fac[0] = fac[1] = facinv[0] = facinv[1] = 1;
        for (var i = 2; i <= n; i++)
        {
            fac[i] = fac[i - 1] * i;
            inv[i] = MOD - inv[MOD % i] * (MOD / i);
            facinv[i] = facinv[i - 1] * inv[i];
        }
    }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt Fac(int n) => fac[n];
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt Inv(int n) => inv[n];
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt FacInv(int n) => facinv[n];
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt Comb(int n, int r)
    {
        if (n < r) return 0;
        if (n == r) return 1;
        return fac[n] * facinv[r] * facinv[n - r];
    }
    public static ModInt Perm(int n, int r)
    {
        if (n < r) return 0;
        return fac[n] * facinv[n - r];
    }

}
#region Template
public partial class Solver
{
    public SC sc = new SC();
    static void Main(string[] args)
    {
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        var sol = new Solver();
        int testcase = 1;
        //testcase = sol.sc.Int;
        //var th = new Thread(sol.Solve, 1 << 26);th.Start();th.Join();
        while (testcase-- > 0)
            sol.Solve();
        Console.Out.Flush();
    }
}
public static class Template
{
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool chmin<T>(ref T a, T b) where T : IComparable<T> { if (a.CompareTo(b) > 0) { a = b; return true; } return false; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool chmax<T>(ref T a, T b) where T : IComparable<T> { if (a.CompareTo(b) < 0) { a = b; return true; } return false; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void swap<T>(ref T a, ref T b) { var t = b; b = a; a = t; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void swap<T>(this IList<T> A, int i, int j) { var t = A[i]; A[i] = A[j]; A[j] = t; }
    public static T[] Create<T>(int n, Func<T> f) { var rt = new T[n]; for (var i = 0; i < rt.Length; ++i) rt[i] = f(); return rt; }
    public static T[] Create<T>(int n, Func<int, T> f) { var rt = new T[n]; for (var i = 0; i < rt.Length; ++i) rt[i] = f(i); return rt; }
    public static void Out<T>(this IList<T> A, out T a) => a = A[0];
    public static void Out<T>(this IList<T> A, out T a, out T b) { a = A[0]; b = A[1]; }
    public static void Out<T>(this IList<T> A, out T a, out T b, out T c) { A.Out(out a, out b); c = A[2]; }
    public static void Out<T>(this IList<T> A, out T a, out T b, out T c, out T d) { A.Out(out a, out b, out c); d = A[3]; }
    public static string Concat<T>(this IEnumerable<T> A, string sp) => string.Join(sp, A);
    public static char ToChar(this int s, char begin = '0') => (char)(s + begin);
    public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> A) => A.OrderBy(v => Guid.NewGuid());
    public static int CompareTo<T>(this T[] A, T[] B, Comparison<T> cmp = null) { cmp = cmp ?? Comparer<T>.Default.Compare; for (var i = 0; i < Min(A.Length, B.Length); i++) { int c = cmp(A[i], B[i]); if (c > 0) return 1; else if (c < 0) return -1; } if (A.Length == B.Length) return 0; if (A.Length > B.Length) return 1; else return -1; }
    public static string ToStr<T>(this T[][] A) => A.Select(a => a.Concat(" ")).Concat("\n");
    public static int ArgMax<T>(this IList<T> A, Comparison<T> cmp = null) { cmp = cmp ?? Comparer<T>.Default.Compare; T max = A[0]; int rt = 0; for (int i = 1; i < A.Count; i++) if (cmp(max, A[i]) < 0) { max = A[i]; rt = i; } return rt; }
    public static T PopBack<T>(this List<T> A) { var v = A[A.Count - 1]; A.RemoveAt(A.Count - 1); return v; }
    public static void Fail<T>(T s) { Console.WriteLine(s); Console.Out.Close(); Environment.Exit(0); }
}
public class Scanner
{
    public string Str => Console.ReadLine().Trim();
    public int Int => int.Parse(Str);
    public long Long => long.Parse(Str);
    public double Double => double.Parse(Str);
    public int[] ArrInt => Str.Split(' ').Select(int.Parse).ToArray();
    public long[] ArrLong => Str.Split(' ').Select(long.Parse).ToArray();
    public char[][] Grid(int n) => Create(n, () => Str.ToCharArray());
    public int[] ArrInt1D(int n) => Create(n, () => Int);
    public long[] ArrLong1D(int n) => Create(n, () => Long);
    public int[][] ArrInt2D(int n) => Create(n, () => ArrInt);
    public long[][] ArrLong2D(int n) => Create(n, () => ArrLong);
    private Queue<string> q = new Queue<string>();
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public T Next<T>() { if (q.Count == 0) foreach (var item in Str.Split(' ')) q.Enqueue(item); return (T)Convert.ChangeType(q.Dequeue(), typeof(T)); }
    public void Make<T1>(out T1 v1) => v1 = Next<T1>();
    public void Make<T1, T2>(out T1 v1, out T2 v2) { v1 = Next<T1>(); v2 = Next<T2>(); }
    public void Make<T1, T2, T3>(out T1 v1, out T2 v2, out T3 v3) { Make(out v1, out v2); v3 = Next<T3>(); }
    public void Make<T1, T2, T3, T4>(out T1 v1, out T2 v2, out T3 v3, out T4 v4) { Make(out v1, out v2, out v3); v4 = Next<T4>(); }
    public void Make<T1, T2, T3, T4, T5>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5) { Make(out v1, out v2, out v3, out v4); v5 = Next<T5>(); }
    public void Make<T1, T2, T3, T4, T5, T6>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5, out T6 v6) { Make(out v1, out v2, out v3, out v4, out v5); v6 = Next<T6>(); }
    public void Make<T1, T2, T3, T4, T5, T6, T7>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5, out T6 v6, out T7 v7) { Make(out v1, out v2, out v3, out v4, out v5, out v6); v7 = Next<T7>(); }
    public (T1, T2) Make<T1, T2>() { Make(out T1 v1, out T2 v2); return (v1, v2); }
    public (T1, T2, T3) Make<T1, T2, T3>() { Make(out T1 v1, out T2 v2, out T3 v3); return (v1, v2, v3); }
    public (T1, T2, T3, T4) Make<T1, T2, T3, T4>() { Make(out T1 v1, out T2 v2, out T3 v3, out T4 v4); return (v1, v2, v3, v4); }
}

#endregion