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: Wrong Answer)

var toInt = x=>parseInt(x,10);
var toIntArr = arr=>arr.map(x=>toInt(x));
var abs = x=>x>0?x:-x;
function min(){var rest = arguments,val=rest[0];for(var i=1;i<rest.length;i++)if(rest[i]<val)val = rest[i];return val}
function max(){var rest = arguments,val=rest[0];for(var i=1;i<rest.length;i++)if(rest[i]>val)val = rest[i];return val}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
 
function Main(input){
	var input = input.split('\n');
	var temp1 = input[0].split(' ');
	var n = toInt(temp1[0]);
	var m = toInt(temp1[1]);
	var abs = [];
	for(var i = 0; i < m; i++){
		c = toIntArr(input[i+1].split(' '));
		a = c[0];
		b = c[1];
		abs[i] = [a,b];
	}
	var l=0;r=n-1;
	var ab;
	var ans = 1;
	for(var i = 0; i < m; i++){
		ab = abs[i];
		if(ab[1]<=l || r<=ab[0]){
			ans++;
			l = ab[0];
			r = ab[1];
		}else{
			l = max(l,ab[0]);
			r = min(r,ab[1]);
		}
	}
	console.log(ans);
}