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<bits/stdc++.h>
using namespace std;
int n,m,num;
struct data
{
	int l,r; 
}a[100100];
bool cmp(data a,data b)
{
	return (a.l<b.l)||(a.l==b.l&&a.r>b.r);
}
int main()
{
	cin>>n>>m;
	for (int i=1;i<=m;i++)
	{
		cin>>a[i].l>>a[i].r;
	}
	sort(a+1,a+1+m,cmp);
	int s=0,l=a[1].l,r=a[1].r;
	a[m+1].l=0;a[m+1].r=n+1;
	for (int i=1;i<=m+1;i++)
	{
		if(a[i].l>=l&&a[i].l<r||a[i].r>l&&a[i].r<=r)
		{
			l=max(l,a[i].l);r=min(r,a[i].r);
		}
		else
		{
		  l=a[i].l;r=a[i].r;num++;
	    }
	}
	cout<<num<<endl;
	return 0;
}