Problem:
Score : 300 points

Problem Statement

We have N ID cards, and there are M gates.

We can pass the i-th gate if we have one of the following ID cards: the L_i-th, (L_i+1)-th, ..., and R_i-th ID cards.

How many of the ID cards allow us to pass all the gates alone?

Constraints

All values in input are integers.

1 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq L_i \leq R_i \leq N

Input

Input is given from Standard Input in the following format:

N M
L_1 R_1
L_2 R_2
\vdots
L_M R_M

Output

Print the number of ID cards that allow us to pass all the gates alone.

Sample Input 1

4 2
1 3
2 4

Sample Output 1

2

Two ID cards allow us to pass all the gates alone, as follows:

The first ID card does not allow us to pass the second gate.

The second ID card allows us to pass all the gates.

The third ID card allows us to pass all the gates.

The fourth ID card does not allow us to pass the first gate.

Sample Input 2

10 3
3 6
5 7
6 9

Sample Output 2

1

Sample Input 3

100000 1
1 100000

Sample Output 3

100000

Solution in JavaScript (node.js v5.12) (judge verdict: Wrong Answer)

'use strict'
function Main(input) {
    // 入力の取得・整形
    input = input.split("\n")
    input = input.map(elem => {
        // 'x y'といった文字列を配列[x, y]に変換する
        return elem.split(' ').map(str => parseInt(str))
    })
    const N = input[0][0]

    // 配列a,bの共通区間を求める関数
    const makeInterval = (a, b) => {
        const start = a[0] > b[0] ? a[0] : b[0]
        const end = a[1] > b[1] ? b[1] : a[1]
        return [start, end]
    }

    // 全区間の共通区間を求めていく
    const resultArr = input.reduce((accum, current, index) => {
        if (index === 0) {
            // 最初は[N, M]だからスキップする
            return accum
        }
        return makeInterval(accum, current)
    }, [1, N])

    // [6, 3]のように、開始のほうが大きくなってる場合は0を返す。そうでない場合区間に含まれる数の個数を返す
    const result = (resultArr[0] > resultArr[1]) ? 0 : resultArr[1] - resultArr[0] + 1

    console.log(result)
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"))