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: Wrong Answer)

#include<iostream>
#include<math.h>
#include<algorithm>
#include<utility>
#include<queue>
#include<string.h>
#include<string>
#include<set>
#include<map>
#include<unordered_map>
#include<vector>
using namespace std;
typedef long long ll;
typedef pair<ll,ll> P;
const ll INF=2e18;
const ll MOD=1e9+7;

ll N,M;
unordered_map<ll,ll> mapA,mapB;
int main(){
    cin>>N>>M;
    for(ll i=0;i<M;i++){
        ll a,b;
        cin>>a>>b;
        mapA[a]++;
        mapB[b]++;
    }
    ll ans=0;
    for(auto o:mapA){
        if(mapB[o.first]!=0){
            ans++;
        }
    }
    cout<<ans+1<<endl;

    return 0;
}