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.Linq;
using BitArray = System.Collections.BitArray;
using BigInteger = System.Numerics.BigInteger;

namespace AtCoderProject
{
    public class Program
    {
        public object Calc()
        {
            var line = consoleReader.SplitInt();
            var N = line[0];
            var M = line[1];
            var ranges = consoleReader.RepeatSplitInt(M).Select(l => new Range { start = l[0], end = l[1] });

            var resRange = new Range { start = 1, end = N };
            foreach (var r in ranges)
            {
                resRange = new Range { start = Math.Max(r.start, resRange.start), end = Math.Min(r.end, resRange.end) };
            }
            if (resRange.start > resRange.end) return 0;
            else return resRange.end - resRange.start + 1;
        }

        struct Range
        {
            public int start;
            public int end;
        }

        public const int mod = 1000000007;
        #region いつもの
        private readonly ConsoleReader consoleReader;
        public Program() : this(new ConsoleReader()) { }
        public Program(ConsoleReader consoleReader) { this.consoleReader = consoleReader; }
        static void Main() => Console.WriteLine(new Program().Calc()); static string MultiLine<T>(IEnumerable<T> source) => string.Join("\n", source);
        #endregion
    }

    #region Console
    public class ConsoleReader { public virtual string String() => Console.ReadLine(); public int Int() => int.Parse(String()); public long Long() => long.Parse(String()); public ulong ULong() => ulong.Parse(String()); public double Double() => double.Parse(String()); public string[] Split() => String().Split().ToArray(); public int[] SplitInt() => String().Split().Select(int.Parse).ToArray(); public long[] SplitLong() => String().Split().Select(long.Parse).ToArray(); public ulong[] SplitULong() => String().Split().Select(ulong.Parse).ToArray(); public double[] SplitDouble() => String().Split().Select(double.Parse).ToArray(); public IEnumerable<string> Repeat(int count) { for (int i = 0; i < count; i++) yield return String(); } public IEnumerable<int> RepeatInt(int count) { for (int i = 0; i < count; i++) yield return Int(); } public IEnumerable<long> RepeatLong(int count) { for (int i = 0; i < count; i++) yield return Long(); } public IEnumerable<string[]> RepeatSplit(int count) { for (int i = 0; i < count; i++) yield return Split(); } public IEnumerable<int[]> RepeatSplitInt(int count) { for (int i = 0; i < count; i++) yield return SplitInt(); } }
    #endregion
}