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)

function main(input) {
    var array = input.split('\n');
    array.shift();
    var bridgeList = [];
    var cut = 1;

    for(var i = 0; i < array.length; i++){
        bridgeList[i]=[];
        for(var j = array[i][0]; j < array[i][2]; j++){
            bridgeList[i].push(parseInt(j));
        }
    }

    for(var i = 0; i < bridgeList.length-1; i++){
        console.log(bridgeList);
        if(sameArray(bridgeList[i], bridgeList[i+1]).length != 0){
            bridgeList[i] = sameArray(bridgeList[i], bridgeList[i+1]);
            bridgeList.splice(i+1, 1);
            i--;
        }
        else{
            cut++;
        }
    }
    console.log(parseInt(cut));
}

function sameArray(arr1, arr2) {
    var newArr = [];
    for(var i = 0 ; i < arr1.length; i++){
      if(arr2.indexOf(arr1[i]) >= 0 ){
        newArr.push(arr1[i]);
      }    
    }
    return newArr;
  }

main(require('fs').readFileSync('/dev/stdin', 'utf8'));