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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.*;
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
	int deltaMin = 100000;
    Set<Integer> nums = new TreeSet<Integer>();
    int n = sc.nextInt();
    int m = sc.nextInt();
    int[][] ab = new int[m][3];
    for (int i = 0; i < m; i++) {
      ab[i][0] = sc.nextInt();
      ab[i][1] = sc.nextInt();
      ab[i][2] = ab[i][1] - ab[i][0];
      deltaMin = Math.min(deltaMin, ab[i][2]);
    }
    for (int i = 0; i < m; i++) {
      if (deltaMin == ab[i][2]) {
        nums.add(ab[i][0]);
      }
    }
    int count = nums.size();
    if (deltaMin != 1) {
      for (int num : nums) {
      	for (int i = 1; i <= deltaMin; i++) {
          if (nums.contains(num + i)) {
          	count--;
          }
      	}
      }
    }
    
    System.out.println(count);

  }
}