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)

///By Stephan Ramirez dsramirezc@unal.edu.co
#include<bits/stdc++.h>
#define ms(a,v) memset(a,v,sizeof a)
#define ll long long
#define N 2005
using namespace std;
int n,m;
int main(){
	#ifdef LOCAL
		freopen("in.txt","r",stdin);
		//freopen("out.txt","w",stdout);
	#endif
    scanf("%d %d",&n,&m);
    vector<pair<int,int> > v;
    for(int i=0;i<m;i++){
        int a,b;
        scanf("%d %d",&a,&b);
        v.push_back(make_pair(a,b));
    }
    sort(v.begin(),v.end());
    int ans=0;
    int l=v[0].first;
    int r=v[0].second;
    for(int i=1;i<m;i++){
        l=v[i].first;
        r=min(r,v[i].second);
        if(l>=r){
            ans++;
            r=v[i].second;
        }
    }
    cout<<ans+1<<endl;
	return 0;
}