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: Wrong Answer)

#include<stdio.h>
#include<stdlib.h>
#define MOD 1000000009+7

int compare(const void *a, const void *b)
{
    return *(int*)a - *(int*)b;
}

int main(){
	int N,M,i,j,pos,ans=0;
	scanf("%d %d",&N,&M);
	int a[M],b[M];
	int aflag=0,bflag=1;
	for(i=0;i<M;i++)scanf("%d %d",&a[i],&b[i]);
	qsort(a,M,sizeof(int),compare);
	qsort(b,M,sizeof(int),compare);
	
	i=0;
	j=0;
	for(pos=1;pos<=N;pos++){
		while(a[i]<=pos){
			if(a[i]==pos)aflag=1;
			i++;
		}
		while(b[j]<=pos){
			if(b[j]==pos)bflag=1;
			j++;
		}
		if(aflag==1 && bflag==1){
			ans++;
			bflag=0;
		}
		aflag=0;
	}
	
	printf("%d\n",ans);
	
	return 0;
}