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<algorithm>
#include<clocale>
#include<cmath>
#include<complex>
#include<csetjmp>
#include<csignal>
#include<cstdarg>
#include<cstddef>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<ctime>
#include<deque>
#include<iostream>
#include<istream>
#include<map>
#include<math.h>
#include<ostream>
#include<queue>
#include<set>
#include<sstream>
#include<string>
#include<vector>
using namespace std;
int n,m,a,b;
vector<int> v,c;
int main(){
	cin>>n>>m;
	for (int i=0;i<m;i++){
		cin>>a>>b;
		v.push_back(a);
		c.push_back(b);
	}
	for (int i=0;i<m;i++){
		int j=i+1,d=c[i];
		while (j<m){
			if (d>v[j]){
				if (d>c[j]) d=c[j];
				c.erase(c.begin()+j);
				v.erase(v.begin()+j);
				m-=1;
				j--;
			}
			j++;
		}
	}
	cout<<m<<endl;
	return 0;
}