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: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Numerics;

namespace AtcoderC
{
    class Program
    {
        /*static void Main(string[] args)
        {
        }*/
        //ABC132C
        //全探索法
        /*static void Main()
        {
            BigInteger N = int.Parse(Console.ReadLine());
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int min = input.Min();
            int max = input.Max();
            int countARC = 0;
            int countABC = 0;
            int Answer = 0;
            for (int i = min; i <= max; i++)
            {
                countABC = 0;
                countARC = 0;
                foreach (int n in input)
                {
                    if (n > i)
                    {
                        countARC++;
                    }
                    else
                    {
                        countABC++;
                    }
                }
                if (countABC == countARC)
                {
                    Answer++;
                }
            }
            Console.WriteLine(Answer);
        }*/
        //2回目
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            Array.Sort(input);
            int min = input[(N / 2) - 1];
            int max = input[N / 2];
            Console.WriteLine(max - min);
        }*/

        //ABC131C
        //ユーグリッドの互除法の応用
        /*static void Main()
        {
            BigInteger[] input = Console.ReadLine().Split().Select(BigInteger.Parse).ToArray();
            BigInteger A = input[0];
            BigInteger B = input[1];
            BigInteger C = input[2];
            BigInteger D = input[3];
            BigInteger ans = (B / C) + (B / D);           
            //最小公倍数
            var ans2 = C * D / lcm(C, D);
            BigInteger ans3 = B / ans2;
            var BB = B - ans + ans3;
            ans = (A - 1) / C + (A - 1) / D;
            ans3 = (A - 1) / ans2;
            var AA = (A - 1) - ans + ans3;
            Console.WriteLine(BB - AA);
            
        }
        static public BigInteger lcm(BigInteger a, BigInteger b)
        {
            var r = a % b;
            while (r != 0)
            {
                a = b;
                b = r;
                r = a % b;
            }
            return b;
        }*/

        //129C
        //動的計画法
        //TLEになる時間がかかり過ぎる
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            List<int> ttt = new List<int>();
            for (int i = 0;i < M;i++)
            {
                var temp = int.Parse(Console.ReadLine());
                ttt.Add(temp);
            }
            int[] dparray = new int[N+2];
            dparray[N] = 1;
            dparray[N - 1] = 1;
            for (int i = N-1; i >= 0; i--)
            {
                if (ttt.Contains(i))
                {
                    dparray[i] = 0;
                }
                dparray[i] = (dparray[i + 1] + dparray[i + 2]) % 1000000007;
            }
            Console.WriteLine(dparray[0]);
        }*/
        //ABC128
        /*static void Main()
        {

        }*/
        //ABC134A
        /*static void Main()
        {
            int r = int.Parse(Console.ReadLine());
            var ans = 3 * r * r;
            Console.WriteLine(ans);
        }*/
        //ABC134B
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int D = input[1];
            int count = 0;
            int ans = N;
            while (true)
            {
                ans = ans - (D * 2 + 1);
                count++;
                if (ans <= 0)
                {
                    break;
                }
            }
            Console.WriteLine(count);
        }*/
        //ABC134C
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            List<int> array = new List<int>();
            for (int i =0;i<N; i++)
            {
                array.Add(int.Parse(Console.ReadLine()));
            }
            List<int> ans = new List<int>();
            int max = array.Max();
            var temp  = new List<int>();
            foreach (int n in array)
            {
                 temp.Add(n);
            }
            temp.Sort();
            temp.Reverse();
            int second = temp[1];

            foreach (int nn in array)
            {
                if (nn == max)
                {
                    Console.WriteLine(second);
                }
                else
                {
                    Console.WriteLine(max);
                }
            }
        }*/
        //134ABCD
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            int count = 0;
            int ans = 0;
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            for (int i = 0;i<N;i++)
            {
                for (int j =0;j<N; j++)
                {
                    if ((j+1)%(i+1)==0)
                    {
                        count += input[j];
                    }
                }
                if (count % 2 == input[i])
                {
                    ans++;
                }
                count = 0;
            }
            if (ans != 0)
            {

            }
        }*/
        //AGC036
        /*static void Main()
        {
            long S = long.Parse(Console.ReadLine());
            long X1 = 1000000000;
            long Y1 = 0;
            /*while (true)
            {
                Y1 = S / X1 + S % X1;
            }
            Y1 = S / X1 + S % X1;
            Console.WriteLine("0 0 " + X1 + " 0 0 " + Y1); 
            /*int tempX = 0;
            List<long> ss = new List<long>();
            while (tempX < S)
            {
                var ans = S % tempX;
                if (ans == 0)
                {
                    ss.Add(ans);
                }
                tempX++;
            }
            Console.WriteLine(tempX);*/
        /*long X1 = 0;
        long Y1 = 1;
        while (true)
        {
            X1 = S % Y1;
            if (X1 <= 1000000000)
            {
                break;
            }
            Y1++;
        }
        Console.WriteLine("0 0 "+X1+" 0 0 "+Y1);
    }*/
        //ABC82A
        /*static void Main()
        {
            double[] input = Console.ReadLine().Split().Select(double.Parse).ToArray();
            double a = input[0];
            double b = input[1];
            double sum = (a + b) / 2;
            Console.WriteLine(Math.Ceiling(sum));

        }*/
        //ABC128C
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            int ans = 0;
            List<int[]> K = new List<int[]>();
            for (int i = 0;i<M;i++)
            {
                var temp = Console.ReadLine().Split().Select(int.Parse).ToArray();
                K.Add(temp);
            }
            int[] P = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int[] light = new int[M];
            for (int i = 0;i < Math.Pow(2,N);i++)
            {
                var isrow = 0;
                var temp = int.Parse(Convert.ToString(i, 2)).ToString("0000000000");
                for (int j  =0;j < M;j++)
                {
                    var ison = 0;
                    for (int k = 0; k < K[j][0];k++)
                    {
                        var index = K[j][k] - 1;
                        if (temp[9-index] == '1')
                        {
                            ison++;
                        }
                    }
                    if (ison % 2 == P[j])
                    {
                        isrow++;
                    }
                    else
                    {
                        break;
                    }
                }
                if (isrow == M)
                {
                    ans++;
                }
            }
            Console.WriteLine(ans);*/
        /*nt[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
        int a = input[0];
        int b = input[1];
        var temp = a * b % 2;
        if (temp == 0)
        {
            Console.WriteLine("even");
        }
        else
        {
            Console.WriteLine("odd");
        }
    }*/
        //ABC127C 
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            List<int[]> gate = new List<int[]>();
            for (int i = 0; i < M; i++)
            {
                var temp = Console.ReadLine().Split().Select(int.Parse).ToArray();
                gate.Add(temp);
            }
            if (M == 1)
            {
                Console.WriteLine(N);

            }
            else
            {
                List<int> card = new List<int>();
                for (int i = gate[0][0]; i <= gate[0][1]; i++)
                {
                    card.Add(i);
                }

                for (int j = 1; j < gate.Count; j++)
                {
                    List<int> ans = new List<int>();
                    var count = 0;
                    for (int i = gate[j][0]; i <= gate[j][1]; i++)
                    {
                        if (card.Contains(i))
                        {
                            count++;
                            ans.Add(i);
                        }
                    }
                    if (count == 0)
                    {
                        break;
                    }
                    card = new List<int>(ans);
                }
                Console.WriteLine(card.Count());
            }
        }*/
        //127C Revenge
        static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            List<int[]> gate = new List<int[]>();
            for (int i = 0; i < M; i++)
            {
                var temp = Console.ReadLine().Split().Select(int.Parse).ToArray();
                gate.Add(temp);
            }
            int Ans = 0;
            for (int i =1;i<=N;i++)
            {
                var count = 0;
                for (int j = 0;j < gate.Count;j++)
                {
                    if (gate[j][0] <= i && i <= gate[j][1])
                    {
                        count++;
                    }  
                }
                if (count == gate.Count)
                {
                    Ans++;
                }
            }
            Console.WriteLine(Ans);
        }
    }
}