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 PyPy3 (7.3.0) (judge verdict: Time Limit Exceeded)

import sys
input = sys.stdin.buffer.readline

N, K = map(int, input().split())

MOD = 998244353



I = [tuple(map(int, input().split())) for _ in range(K)]

class BinaryIndexedTree():  #1-indexed
    def __init__(self, n):
        self.n = n
        self.tree = [0 for _ in range(n + 1)]

    def sum(self, idx):
        res = 0
        while idx:
            res += self.tree[idx]
            res %= MOD
            idx -= idx & -idx
        return res

    def add(self, idx, x):
        while idx <= self.n:
            self.tree[idx] += x
            self.tree[idx] % MOD
            idx += idx & -idx

    def bisect_left(self, x):
        if x <= 0: return 0
        res, tmp = 0, 2**((self.n).bit_length() - 1)
        while tmp:
            if res + tmp <= self.n and self.tree[res + tmp] < x:
                x -= self.tree[res + tmp]
                res += tmp
            tmp >>= 1
        return res + 1

class RAQandRSQ():
    def __init__(self, n):
        self.bit1 = BinaryIndexedTree(n)
        self.bit2 = BinaryIndexedTree(n)

    def add(self, lt, rt, x):
        self.bit1.add(lt, -x * (lt - 1))
        self.bit1.add(rt, x * (rt - 1))
        self.bit2.add(lt, x)
        self.bit2.add(rt, -x)

    def sum(self, lt, rt):
        rsum = self.bit2.sum(rt - 1) * (rt - 1) + self.bit1.sum(rt - 1)
        lsum = self.bit2.sum(lt - 1) * (lt - 1) + self.bit1.sum(lt - 1)
        return rsum - lsum

R = RAQandRSQ(N)
R.add(1, 2, 1)

for i in range(1, N):
    s = R.sum(i, i + 1)
    for l, r in I:

        R.add(min(i + l, N + 1), min(i + r + 1, N + 1), s)

print(R.sum(N, N + 1) % MOD)