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 9.2.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>

typedef struct List {
	struct List *next;
	int v;
} list;

int main()
{
	int i, u, w, N, M;
	scanf("%d %d", &N, &M);
	list **adj = (list**)malloc(sizeof(list*) * (N + 1)), *d = (list*)malloc(sizeof(list) * M * 2);
	for (i = 1; i <= N; i++) adj[i] = NULL;
	for (i = 0; i < M; i++) {
		scanf("%d %d", &u, &w);
		d[i*2].v = w;
		d[i*2+1].v = u;
		d[i*2].next = adj[u];
		d[i*2+1].next = adj[w];
		adj[u] = &(d[i*2]);
		adj[w] = &(d[i*2+1]);
	}
	
	int ans = 0, l;
	list *p;
	for (i = 2, l = 1; i <= N; i++) {
		for (p = adj[i]; p != NULL; p = p->next) {
			if (p->v >= l && p->v < i) {
				ans++;
				l = i;
				break;
			}
		}
	}
	
	printf("%d\n", ans);
	fflush(stdout);
	return 0;
}