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: Accepted)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <math.h>

int main(void){
    int n,m,near=0,ans=0;
    scanf("%d%d",&n,&m);
    int a[m],b[m],c[n];
    for (int i=0; i<n; i++){
        c[i]=1e9;
    }
    for (int i=0; i<m; i++){
        scanf("%d%d",&a[i],&b[i]);
        a[i]--;
        b[i]--;
        if(c[a[i]]>b[i]){
            c[a[i]]=b[i];
        }
    }
    for (int i=0; i<n; i++){
        if (c[i]!=1e9){
            if (i>=near){
                ans++;
                near=c[i];
            } else if (c[i]<near){
                near=c[i];
            }
        }
    }
    printf("%d\n",ans);
	return 0 ;
}