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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<cstring>
#include<iostream>
#include<algorithm>

using namespace std;

struct node{
	int l,r;
}a[100010];

bool cmp(node a,node b){
	return a.l<b.l;
}

int main()
{
	int n,m;
	cin>>n>>m;
	int ans=0;
	for(int i=1;i<=m;i++){
		cin>>a[i].l>>a[i].r;
	}
	
	sort(a+1,a+1+m,cmp);
	int l=a[1].l,r=a[1].r;
	ans=1;
	for(int i=2;i<=m;i++){
		if(a[i].l>l) l=a[i].l;
		if(a[i].r<r) r=a[i].r;
		if(l>=r){
			ans++;
			l=a[i].l;
			r=a[i].r;
		}
	}
	cout<<ans<<endl;
	return 0;
}