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 <cstdio>
#include <algorithm>
using namespace std;
struct me{
    int x,y;
}v[100005];
bool com(struct me a, struct me b){
    return a.x<b.x;
}
int main(){
    int n,q;
    scanf("%d%d",&n,&q);
    for(int i=1;i<=q;i++){
        scanf("%d%d",&v[i].x,&v[i].y);
        if(v[i].x>v[i].y)
            swap(v[i].x,v[i].y);
    }
    sort(v+1,v+1+q,com);
    int s=0;
    int p=0xfffffff,j=1;
    for(int i=1;i<=n;i++){
        if(i==p) {
            s++;
            p=0xfffffff;
        }
        for(j;v[j].x==i;j++){
            p=min(v[j].y,p);
        }

    }
    if(p!=0xfffffff)
        s++;
    printf("%d\n",s);
}