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: Runtime Error)

#include <stdio.h>

main(){
    int N,M;
    scanf("%d %d",&N,&M);

    int y[100001];
    int i,j,k;
    for(i=1;i<=N;i++)
        y[i]=2*N;

    int a,b;

    for(i=0;i<M;i++){
        scanf("%d %d",&a,&b);
        if(b<y[a])
            y[a]=b;
    }
    for(i=1;i<N;i++){
        if(y[i]==2*N)continue;
        for(k=i+1;k<y[i];k++){
            if(y[k]<=y[i])//ganni
                y[i]=2*N;
            else{   //and
                y[k]=y[i];
                y[i]=2*N;
                break;
            }
        }
    }
    int rt=0;
    for(i=1;i<=N;i++)
        if(y[i]!=2*N)
            rt++;
    printf("%d\n",rt);

    return 0;
}