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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

/*
csc -debug C.cs && mono --debug C.exe <<< "4 2
1 3
2 4"
*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Reflection;

public class HelloWorld {
    public static void SolveCodeForces() {
        var N = cin.NextInt();
        var M = cin.NextInt();

        var min = 1;
        var max = N;

        for (var i = 0; i < M; i++) {
            var l = cin.NextInt();
            var r = cin.NextInt();
            min = Math.Max(min, l);
            max = Math.Min(max, r);
        }

        System.Console.WriteLine(Math.Max(0, max - min + 1));
    }

    static Scanner cin = new Scanner();
    static StringBuilder cout = new StringBuilder();

    static public void Main () {
        SolveCodeForces();
        // IncreaseStack(SolveCodeForces);
        // System.Console.Write(cout.ToString());
    }

    public static void IncreaseStack(ThreadStart action, int stackSize = 128000000)
    {
        var thread = new Thread(action, stackSize);
        thread.Start();
        thread.Join();
    }
}

public static class Helpers {
    public static T[] CreateArray<T>(int length, Func<int, T> itemCreate) {
        var result = new T[length];
        for (var i = 0; i < result.Length; i++)
            result[i] = itemCreate(i);
        return result;
    }
}

public class Scanner
{
    private string[] line = new string[0];
    private int index = 0;

    public string Next() {
        if (line.Length <= index) {
            line = Console.ReadLine().Split(' ');
            index = 0;
        }
        var res = line[index];
        index++;
        return res;
    }

    public int NextInt() {
        return int.Parse(Next());
    }

    public long NextLong() {
        return long.Parse(Next());
    }

    public ulong NextUlong() {
        return ulong.Parse(Next());
    }

    public string[] Array() {
        line = Console.ReadLine().Split(' ');
        index = line.Length;
        return line;
    }

    public int[] IntArray(int emptyPrefixLen = 0) {
        var array = Array();
        var result = new int[emptyPrefixLen + array.Length];
        for (int i = 0; i < array.Length; i++)
            result[emptyPrefixLen + i] = int.Parse(array[i]);
        return result;
    }

    public long[] LongArray() {
        var array = Array();
        var result = new long[array.Length];
        for (int i = 0; i < array.Length; i++)
            result[i] = long.Parse(array[i]);
        return result;
    }

    public ulong[] UlongArray() {
        var array = Array();
        var result = new ulong[array.Length];
        for (int i = 0; i < array.Length; i++)
            result[i] = ulong.Parse(array[i]);
        return result;
    }
}