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 Python (3.4.3) (judge verdict: Wrong Answer)

# -*- coding: utf-8 -*-
"""
Created on Sun May 19 20:12:17 2019

@author: Owner
"""

import numpy as np
from operator import itemgetter
import bisect

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


ab = [(0, 0)]*M
c = np.zeros(N)
for m in range(M):
    ab[m] = tuple(map(int, input().split()))

ab = tuple(sorted(ab, key = itemgetter(0)))

a = [0]*M
b = [0]*M
for m in range(M):
    a[m], b[m] = ab[m]
b.sort()
res = 0
n = -1
while True:
    tmp = bisect.bisect_left(a, n)
    
    n = b[bisect.bisect_right(b, a[tmp])]
    res+=1
    if n > a[m]:
        break
    
    
        

print(res)