Problem:
Score : 400 points

Problem Statement

There are N islands lining up from west to east, connected by N-1 bridges.

The i-th bridge connects the i-th island from the west and the (i+1)-th island from the west.

One day, disputes took place between some islands, and there were M requests from the inhabitants of the islands:

Request i: A dispute took place between the a_i-th island from the west and the b_i-th island from the west. Please make traveling between these islands with bridges impossible.

You decided to remove some bridges to meet all these M requests.

Find the minimum number of bridges that must be removed.

Constraints

All values in input are integers.

2 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq a_i < b_i \leq N

All pairs (a_i, b_i) are distinct.

Input

Input is given from Standard Input in the following format:

N M
a_1 b_1
a_2 b_2
:
a_M b_M

Output

Print the minimum number of bridges that must be removed.

Sample Input 1

5 2
1 4
2 5

Sample Output 1

1

The requests can be met by removing the bridge connecting the second and third islands from the west.

Sample Input 2

9 5
1 8
2 7
3 5
4 6
7 9

Sample Output 2

2

Sample Input 3

5 10
1 2
1 3
1 4
1 5
2 3
2 4
2 5
3 4
3 5
4 5

Sample Output 3

4

Solution in PyPy3 (2.4.0) (judge verdict: Wrong Answer)

from copy import copy
from collections import deque

N,M=map(int,input().split())
ab=[]
for i in range(M):
    a,b=map(int,input().split())
    ab.append([a,b])

"""
N=10**5
M=10**5
"""
ab=[]
for i in range(1,N):
    ab.append([i,i+1])

AB=copy(ab)
ab.sort(key=lambda x :x[1])
AB.sort()
ab=deque(ab)
AB=deque(AB)
num=0
while(ab):
    p=ab.popleft()
    AB.remove(p)
    num+=1
    e=p[1]
    Q=copy(AB)
    for I in AB:
        if I[0]<e:
            Q.remove(I)
            ab.remove(I)
        else:
            break
    AB=Q
print(num)