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>
#include <string.h>
#include <math.h>

//qsort(vector, n, sizeof(int), fw);

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

int main()
{
	int n, m, ans = 0, cnt = 0;
	int a[100010], b[100010];
	int c[200010];
	scanf("%d%d", &n, &m);
	for (long int i = 0; i < m; i++)
	{
		scanf("%d%d", &a[i], &b[i]);
		c[i] = b[i];
	}
	qsort(c, m, sizeof(int), fw);
	for (int i = 0; i < m; i++)
	{
      int f=0;
		for (int j = 0; j < m; j++)
		{
			if (a[j] < c[i]) {
				a[j] = n+1;
				cnt++;
              f++;
			}
		}
		if(f)ans++;
		if (cnt == m)break;
	}
	printf("%d", ans);
	return 0;
}