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 "bits/stdc++.h"
#include <set>
using namespace std;
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define RFOR(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define rep(i, n) for (int i = 0; i < (n); i++)
#define rep1(i, n) for (int i = 1; i < (n); i++)
#define rrep(i, n) for (int i = (n)-1; i >= 0; i--)
#define all(i) (i).begin(), (i).end()
#define rall(i) (i).begin(), (i).end(), greater<int>()
//#define int long long
#define elif else if
#define eb emplace_back
#define pb push_back
#define mp make_pair
#define fst first
#define sec second
#define mod 1000000007
template <typename T>

void print(T x)
{
	std::cout << x << " ";
}
template <typename T>

void println(T x)
{
	std::cout << x << "\n";
}

typedef pair <int, int>P;

int main() {
	int N, M;
	cin >> N >> M;
	vector<P>a(M);
	rep(i, M) {
		cin >> a[i].second >> a[i].first;
	}
	sort(a.begin(), a.end());
	int end = 0;
	int ans = 0;
	rep(i, M) {
		if (a[i].second >= end) {
			ans++;
			end = a[i].first;
		}
	}
	print(ans);
}