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 Java8 (OpenJDK 1.8.0) (judge verdict: Time Limit Exceeded)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

public class Main {

	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] tmp = br.readLine().split(" ");
		int N = Integer.parseInt(tmp[0]);
		int M = Integer.parseInt(tmp[1]);

		List<Integer> L = new ArrayList<Integer>();
		List<Integer> R = new ArrayList<Integer>();

		for(int i = 0;i<M;i++){
			tmp = br.readLine().split(" ");
			L.add(Integer.parseInt(tmp[0]));
			R.add(Integer.parseInt(tmp[1]));
		}

		//1つ目の範囲を代入

		List<Integer> range = new ArrayList<Integer>();
		List<Integer> n_range = new ArrayList<Integer>();

		for(int i = L.get(0); i<=R.get(0);i++){
			range.add(i);
		}

		//2つ目以降は取り除いていく
		int max = range.get(range.size()-1);
		int min = range.get(0);

		for(int i = 1;i<M;i++){
			for(int j = L.get(i);j<=R.get(i);j++){
				if(j>max)break;
				if(R.get(R.size()-1) < min)break;
				if(range.contains(j))n_range.add(j);
			}
			range = new ArrayList<Integer>(n_range);
			n_range.clear();
		}

		System.out.println(range.size());


	}
}