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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <cstdio>
#include <cstring>
#include <vector>
#include <algorithm>

using namespace std;

pair <int, int> a[123456];

int main (void)
{
  int n, i, m;
  while (scanf ("%d %d", &n, &m) != EOF)
  {
    for (i = 0; i < m; i++)
      scanf ("%d %d", &a[i].first, &a[i].second);
    sort (a, a + m);
    int res = 0, curb = 0, cure = 123456;
    for (i = 0; i < m; i++)
    {
      curb = a[i].first;
      cure = min (cure, a[i].second);
      if (cure <= curb)
      {
        res++;
        cure = a[i].second;
      }
    }
    if (curb < cure)
      res++;
    printf ("%d\n", res);
  }
  return 0;
}