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>

int main(void) {
  int N, M;
  scanf("%d%d", &N, &M);
  int ab[2][M];
  int i, j;
  int t;
  int B[N - 1];
  int r;
  int a = 0;
  for(i = 0; i < N - 1; i++) B[i] = 1;
  for(i = 0; i < M; i++) {
    scanf("%d%d", &ab[0][i], &ab[1][i]);
    for(j = 0; j < i; j++) if(ab[1][i] < ab[1][j]) t = ab[1][i] , ab[1][i] = ab[1][j], ab[1][j] = t;
  }
  for(i = 0; i < M; i++) {
    r = 0;
    for(j = ab[0][i] - 1; j < ab[1][i] - 2; j++) {
      if(!B[j]) {
        r = 1;
        break;
      }
    }
    if(!r) {
      a++;
      B[j] = 0;
    }
  }
  printf("%d\n", a);
  return 0;
}