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

typedef struct war{
  int a;
  int b;
}War;

//昇順
int acs(const void *a, const void *b) {
  if (((War *)a)->a == ((War *)b)->a) return ((War *)a)->b - ((War *)b)->b;
  return ((War *)a)->a - ((War *)b)->a;
}

int main() {
  int n, m;
  scanf("%d%d", &n, &m);
  War w[m];
  for (int i=0; i<m; i++) scanf("%d%d", &(w[i].a), &(w[i].b));
  int ans = 1, min = w[0].b;
  qsort(w, m, sizeof(War), acs);
  for (int i=0; i<m; i++) {
    if (w[i].a < min) {
      min = (min > w[i].b) ? w[i].b : min;
      continue;
    }
    ans++;
    min = w[i].b;
  }
  printf("%d\n", ans);
  return 0;
}