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 Python (3.8.2) (judge verdict: Accepted)

P = 998244353

def val(x):
	return (x+P)%P

n, k = map(int,input().split())
plages = []
for _ in range(k):
	x,y=map(int,input().split())
	plages+=[[x,y]]
#plages.sort()

pref = [0]*(n+1)
ans = [0]*(n+1)
ans[1] = pref[1] = 1

for i in range(2,n+1):
	for p in plages:
		l = max(0,i-p[1])
		r = max(0,i-p[0])
		add= 0
		if l==0 and r==0:add=0
		elif l==0: add = pref[r]
		else:
			add = val(pref[r]-pref[l-1])
		# ~ print("i = ",i,"p = ",p,"l = ",l, "r = ",r, "add = ",add)
		ans[i] += add
		ans[i]%=P
	pref[i] = pref[i-1]+ans[i]
	pref[i]%=P
	
# ~ print("pref : ",pref)
# ~ print("ans  : ",ans)

print(ans[n])