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>

struct pair{
	int a;
	int b;
};
int compare_int(const void *a, const void *b)
{
	return ((struct pair*)a)->b - ((struct pair*)b)->b;
}

int main(void){
	int n,m;
	scanf("%d %d",&n,&m);
	int i,j;
	struct pair arr[m];
	for(i=0;i<m;i++){
		scanf("%d %d",&arr[i].a,&arr[i].b);
	}
	qsort(arr,m,sizeof(struct pair), compare_int);
	int max = 0;
	int ans = 0;
	for(i=0;i<m;i++){
		if(max<arr[i].a){
			max = arr[i].b -1;
			ans ++;
		}
	}
	/*printf("--------\n");
	for(i=0;i<m;i++){
		printf("%d %d\n",a[i],b[i]);
	}*/
	printf("%d",ans);
	return 0;
}