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: Wrong Answer)

#include<bits/stdc++.h>

#define INF 1e9
#define llINF 1e18
#define MOD 1e9+7
#define pb push_back
#define mp make_pair 
#define F first
#define S second
#define ll long long
using namespace std;
int main(){
  int n,m;cin>>n>>m;
  pair<int,int> rui[n+100]={};
  for(int i=0;i<n+10;i++)
    rui[i]=mp(0,0);
  ll ans=0;
  for(int i=0;i<m;i++){
    int a,b;cin>>a>>b;
    rui[a-1].F++;
    rui[b-1].S++;
  }
  int minus=0;
  int now=0;
  for(int i=0;i<n;i++){
    if(minus>0&&rui[i].S>0){
      minus-=rui[i].S;
    }else if(now>0&&rui[i].S>0){
      now=0;
      minus+=now-1;
      ans++;
    }
    now+=rui[i].F;
  }
  cout<<ans<<endl;
  return 0;
}