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 <vector>
#include <algorithm>

using namespace std;

int main() {
	int n, m;
	cin >> n >> m;

	vector<vector<int>> v(m, vector<int>(2));
	
	for (auto &&e : v) {
		cin >> e[0] >> e[1];
	}
	
	sort(v.begin(), v.end(), [](auto& a, auto& b) { return a[0] < b[0]; });
	
	int count = 1, tb = 0, te = n;
	for (int i = 0; i < m; ++i) {
		if (tb < v[i][0]) {
			tb = v[i][0];
			
			if (tb >= te) {
				++count;
				te = n;
			}
		}
		
		if (te > v[i][1]) {
			te = v[i][1];
		}
	}
	
	cout << count << endl;
}