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 M 100000

int aa[M], bb[M];

int compare(const void *a, const void *b) {
	int i = *(int *) a;
	int j = *(int *) b;

	return aa[i] != aa[j] ? aa[i] - aa[j] : bb[j] - bb[i];
}

int main() {
	static int ii[M];
	int n, m, i, b, ans;

	scanf("%d%d", &n, &m);
	for (i = 0; i < m; i++) {
		scanf("%d%d", &aa[i], &bb[i]);
		ii[i] = i;
	}
	qsort(ii, m, sizeof *ii, compare);
	ans = 1;
	b = n + 1;
	for (i = 0; i < m; i++) {
		if (aa[ii[i]] >= b) {
			ans++;
			b = n + 1;
		}
		if (b > bb[ii[i]])
			b = bb[ii[i]];
	}
	printf("%d\n", ans);
	return 0;
}