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 JavaScript (node.js v5.12) (judge verdict: Accepted)

function Main(s) {
  var s = s.split("\n").map(e => e.split(" ").map(e => e*1));
  n = s[0][0], m = s[0][1];
  a_b = s.slice(1, m+1).sort((a, b) => a[1]-b[1]);
  var end = a_b[0][1];
  var ans = 1;
  for (var i = 0; i < m; i++) {
    if (end <= a_b[i][0]) {
      ans++;
      for (var j = i; j < m; j++) {
        if (end < a_b[j][1]) {
          end = a_b[j][1];
          break;
        }
      }
    }
  }
  console.log(ans);
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));