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 <iostream>
#include <list>
#include <utility>

int main(){
  using namespace std;
  int M, N;
  std::cin >> N >> M;
  list<pair<int, int> > v;
  for(size_t i = 0; i < M; ++i){
  	int a, b;
    std::cin >> a >> b;
    v.push_back(make_pair(a, b));
  }
  size_t count = 0;
  while(!v.empty()){
    ++count;
  	int a_max = 0;
    for(auto p : v)
    	a_max = max(a_max, p.first);
   	for(auto iter = v.begin(); iter != v.end();){
      if(iter->second > a_max) v.erase(iter++);
      else ++iter;
    }
  }
  std::cout << count << "\n";

}