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)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Linq.Expressions;
using System.Text;

class Procon {
    static void Main (string[] agrs) {
        var N = Int();
        var M = Int();

        var maxL = 0;
        var minR = 10000000;
        for ( int i = 0; i < M; i++ ) {
            var L = Int();
            var R = Int();
            maxL = Math.Max(maxL, L);
            minR = Math.Min(minR, R);
        }

        if (maxL > minR) {
            Console.WriteLine(0);
        } else {
            Console.WriteLine(minR - maxL + 1);
        }
    }

    static string String () => Scanner.nextString ();
    static int Int () => int.Parse (Scanner.nextString ());
    static long Long () => long.Parse (Scanner.nextString ());
    static double Double () => double.Parse (Scanner.nextString ());
    static char[] Chars () => String ().ToCharArray ();
    static string[] Strings () => Console.ReadLine ().Split (' ');
    static int[] Ints () => Strings ().Select (v => int.Parse (v)).ToArray ();
    static long[] Longs () => Strings ().Select (v => long.Parse (v)).ToArray ();
    static double[] Doubles () => Strings ().Select (v => double.Parse (v)).ToArray ();
    const int M = 1000000007;
}

class PriorityQueue<T> where T : IComparable<T> {
    public T[] heap;
    public int size;
    public int sign;
    public PriorityQueue (int N, bool descend = false) {
        heap = new T[N];
        sign = 1;
        if (descend) sign = -1;
    }
    public int Compare (T x, T y) {
        return x.CompareTo (y) * sign;
    }
    public void Push (T x) {
        int i = size++;
        while (i > 0) {
            int p = (i - 1) / 2;
            if (Compare (x, heap[p]) >= 0) {
                break;
            }
            heap[i] = heap[p];
            i = p;
        }
        heap[i] = x;
    }
    public T Pop () {
        T ret = heap[0];
        T x = heap[--size];
        int i = 0;
        while (i * 2 + 1 < size) {
            int a = i * 2 + 1;
            int b = i * 2 + 2;
            if (b < size && Compare (heap[a], heap[b]) > 0) {
                a = b;
            }
            if (Compare (heap[a], x) >= 0) {
                break;
            }
            heap[i] = heap[a];
            i = a;
        }
        heap[i] = x;
        return ret;
    }
    public int Count () {
        return size;
    }
}

class Scanner {
    static string[] s = new string[0];
    static int i = 0;
    static int max_i = 0;
    static public string nextString () {
        if (i >= s.Length) {
            s = Console.ReadLine ().Split (' ');
            max_i = s.Length;
            i = 0;
            if (max_i == 0) {
                return "";
            }
            return s[i++];
        }
        return s[i++];
    }
}