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 <bits/stdc++.h>
using namespace std;
#define __ <<' '<<
#define ___ <<' '
#define bash push_back
#define ALL(x) x.begin(),x.end()
//#define int long long

struct IoSetup {IoSetup() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout <<fixed<<setprecision(12);
    cerr <<fixed<<setprecision(12);
}}IoSetup;

using ll = long long;
using pii = pair<int, int>;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr int SMOD = 1000000007;
constexpr int NMOD = 998244353;
constexpr int dx[]={1,0,-1,0,1,1,-1,-1};
constexpr int dy[]={0,1,0,-1,-1,1,-1,1};

inline bool inside(int x,int y,int w,int h){return (x>=0 && y>=0 && x<w && y<h);}
template<class T>bool chmax(T &a, const T &b){return(a<b?a=b,true:false);}
template<class T>bool chmin(T &a, const T &b){return(a>b?a=b,true:false);}
inline int qp(int a,ll b,int mo){ll ans=1;do{if(b&1)ans=1ll*ans*a%mo;a=1ll*a*a%mo;}while(b>>=1);return ans;}



signed main() {

    int n, m;
    cin >> n >> m;

    int a, b;
    priority_queue<pii, vector<pii>, greater<pii>> que;

    for(int i = 0; i < m; i++) {
        cin >> a >> b;
        que.push({a, 1});
        que.push({b, 0});
    }

    int c = 0;
    int ans = 0;
    while(!que.empty()) {
        auto p = que.top();
        que.pop();
        
        if(p.second) c++;
        else if(c) {
            ans++;
            c = 0;
        }
    }

    cout << ans << endl;

  
    return 0;
}