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<cstdio>
 
#define gc getchar
#define db double
#define ll long long int
#define ull unsigned long long int
#define sc(x) scanf("%d", &x)
#define scl(x) scanf("%lld", &x)
#define sclu(x) scanf("%llu", &x)
#define scd(x) scanf("%lf", &x)
#define pass cout<<"pass\n"
#define vi vector<int>
#define pb push_back
#define vec vector
#define vi vector<int>
#define el "\n"
#define pii pair<int, int>
#define F first
#define S second
#define mp make_pair
#define mt make_tuple
#define f(i,x,y) for(int i=x;i<y;++i)
#define ff(i,x,y) for(int i=x;i<=y;++i)
#define MOD 1000000007
#define MAX2 1000001
#define MAX 103
#define pbb pair<bool,bool>

using namespace std;

int n,m;

bool comp(pii a, pii b) {
	return (a.S < b.S);
}

int main() {
	int x,y;
	sc(n); sc(m);
	vec<pii> ar(m);
	f(i,0,m) {
		sc(ar[i].F); sc(ar[i].S);
	}
	ll ans =0LL;
	int p=-1;
	sort(ar.begin(), ar.end(), comp);
	for(int i=0;i<m;++i) {
	//	cout << "ar = " << ar[i].F << " " << ar[i].S << el;
		if(ar[i].F < p) continue;
		ans+= 1;
		p = ar[i].S;
	//	cout << "i = " << i << " ans = " << ans << el;
	}
	cout << ans << el;
	ar.clear();
	return 0;
}