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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>
#define ll long long
#define rep(i,l,r)for(ll i=(l);i<(r);i++)

typedef struct pair{ll x,y;}P;
int pcmp(const void*a,const void*b){
	if((*(P*)a).y<(*(P*)b).y)return -1;
	return 1;
}

P kukan[100010];

int main(){
	ll n,m;
	scanf("%lld%lld",&n,&m);
	rep(i,0,m){
		ll x,y;
		scanf("%lld%lld",&x,&y);
		kukan[i].x=x;
		kukan[i].y=y;
	}
	qsort(kukan,m,sizeof(P),pcmp);
	
	ll pre=-1;
	ll ans=0;
	rep(i,0,m){
		if(pre<kukan[i].x){
			ans++;
			pre=kukan[i].y-1;
		}
	}
	printf("%lld\n",ans);
	return 0;
}