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)

"use strict"

function main(input) {
    let elims = []
    let lastelim = 0
    let arry = input.split('\n').map(x => x.split(' ').map(Number))
    let meta = arry.shift()
    //sort asc by b
    arry = arry.sort((x, y) => x[1] - y[1]) // 昇順にアソート
    let o
    for (let i in arry) {
        o = arry[i]
        if (lastelim < o[0]) {
            lastelim = o[1] - 1
            elims.push(lastelim)
        }
    }

    console.log(elims.length);

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