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<algorithm>
#include<numeric>
#include<vector>
#include<set>
#include<map>
#include<math.h>
#include<queue>
#define cinf(n,x,y) for(int i=0;i<(n);i++) cin >> x[i]>>y[i];
typedef long long int ll;
long long GCD(long long a, long long b) { return b ? GCD(b, a%b) : a; }
using namespace std;

int main(){
  int n,m; cin >>n>>m;
  int a,b;
  vector<pair<int,int>> v;
  for(int i=0;i<m;i++){
    cin>>a>>b;
    v.push_back(make_pair(b,a));
  }
  sort(v.begin(),v.end());
  int cnt=0;
  int check=-1;
  for(int i=0;i<m;i++){
    if(v[i].second>check){
      cnt++;
      check=v[i].first-1;
    }
  }
  cout <<cnt<<endl;
}