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>
using namespace std;
using ll = long long;
#define pb push_back
#define pii pair<ll,ll>
#define ff first
#define ss second
#define mxx 100005
#define endl "\n"
const ll mod=1e9+7;

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

    vector<ll>v[n+5];
    for(int i=0; i<m; i++)
    {
        ll a,b;
        cin>>a>>b;
        v[a].pb(b);
    }

    ll ans=0,mn=1e9;
    for(int i=1; i<=n; i++)
    {
        if(i==mn)
        {
            ans++;
            mn=1e9;
        }
        if(v[i].size())
        {
            for(auto it:v[i])
                mn=min(mn,it);
        }
    }

    cout<<ans<<endl;

    return 0;
}