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: Wrong Answer)

#include <malloc.h>
#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

//ABC103D
int compare_int(const void *a, const void *b) { //sort
  return *(int*) a-*(int*) b;
}

int main(void) {
  int n, m, k=0, i, cou=0, tmp;
  scanf(" %d %d", &n, &m);
  int a[m], b[m];
  for(i=0; i<m; i++){
    scanf(" %d %d", &a[i], &b[i]);
  }

  qsort(a, m, sizeof(int), compare_int); //sort
  qsort(b, m, sizeof(int), compare_int); //sort
  for(i=0; i<m; i++){
    if(b[i]>a[k]){
      while(b[i]>a[k]&&m>k++);
      cou++;
    }
    if(k>=m) break;
  }
  printf("%d", cou);
}