D - Islands War

Time Limit: 2 sec / Memory Limit: 1024 MB

### 問題文

i 番目の橋は、西から i 番目の島と西から i+1 番目の島を接続しています。

ある日、いくつかの島同士で争いが起こり、島の住人たちから M 個の要望がありました。

あなたは橋をいくつか取り除くことでこれら M 個の要望全てを叶えることにしました。

### 制約

• 入力は全て整数である
• 2 \leq N \leq 10^5
• 1 \leq M \leq 10^5
• 1 \leq a_i < b_i \leq N
• (a_i, b_i) は全て異なる

### 入力

N M
a_1 b_1
a_2 b_2
:
a_M b_M


### 入力例 1

5 2
1 4
2 5


### 出力例 1

1


### 入力例 2

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


### 出力例 2

2


### 入力例 3

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


### 出力例 3

4


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