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 (GCC 5.4.1) (judge verdict: Time Limit Exceeded)

//d.c D-Islands War
#include<stdio.h>
#include<stdbool.h>

void iswap(int a[],int i,int j){
    int x = a[i];
    a[i] = a[j];
    a[j] = x;
}

void bubblesort(int n,int a[],int b[]){
    bool done = false;
    while(!done){
        done = true;
        for(int i=1;i<n;++i){
            if(a[i-1]>a[i]){
                iswap(a,i-1,i);
                iswap(b,i-1,i);
                done = false;
            }
        }
    }
}


int main(void){
    int n,m,x=-1,ans=0;
    scanf("%d %d",&n,&m);
    int a[m],b[m];
    for(int i=0;i<m;++i){
        scanf("%d %d",&a[i],&b[i]);
    }
    bubblesort(m,b,a);

    for(int i=0;i<m;++i){
        if(a[i]<=x){
            //do nothing
        }else{
            x = b[i] - 1;
            ans++;
        }
    }

    printf("%d\n",ans);
    return 0;
}