using System;
using System.IO;

namespace abc127
{
    class Program
    {
        static void Main(string[] args)
        {
            var scanner = new Scanner();
            var n = scanner.GetNextAsInt();
            var m = scanner.GetNextAsInt();

            var min = 1;
            var max = n;

            for (var im = 0; im < m; im++)
            {
                var l = scanner.GetNextAsInt();
                var r = scanner.GetNextAsInt();
                min = Math.Max(min, l);
                max = Math.Min(max, r);
            }

            var result = max - min + 1;
            if (result < 0)
            {
                result = 0;
            }
            Console.WriteLine(result);
        }
    }

    #region String Scanner
    public class Scanner
    {
        private const char Separator = ' ';
        private readonly StreamReader _Stream = new StreamReader(Console.OpenStandardInput());
        private int _Pos;
        private string[] _Line = new string[0];

        #region get a element
        public string GetNext()
        {
            // ReSharper disable once InvertIf
            if (this._Pos >= this._Line.Length)
            {
                // ReSharper disable once PossibleNullReferenceException
                this._Line = this._Stream.ReadLine().Split(Separator);
                this._Pos = 0;
            }
            return this._Line[this._Pos++];
        }
        public int GetNextAsInt()
        {
            return int.Parse(this.GetNext());
        }
        public long GetNextAsLong()
        {
            return long.Parse(this.GetNext());
        }
        public double GetNextAsDouble()
        {
            return double.Parse(this.GetNext());
        }
        #endregion

        #region get array
        public string[] ToArrayFromNextLine()
        {
            if (this._Pos >= this._Line.Length)
            {
                // ReSharper disable once PossibleNullReferenceException
                this._Line = this._Stream.ReadLine().Split(Separator);
            }

            this._Pos = this._Line.Length;
            return this._Line;
        }
        public int[] ToIntArrayFromNextLine()
        {
            return ToIntArray(this.ToArrayFromNextLine());
        }
        public long[] ToLongArrayFromNextLine()
        {
            return ToLongArray(this.ToArrayFromNextLine());
        }
        public double[] ToDoubleArrayFromNextLine()
        {
            return ToDoubleArray(this.ToArrayFromNextLine());
        }
        #endregion

        #region convert array
        private static int[] ToIntArray(string[] array)
        {
            var result = new int[array.Length];
            for (var i = 0; i < array.Length; i++)
            {
                result[i] = int.Parse(array[i]);
            }
            return result;
        }
        private static long[] ToLongArray(string[] array)
        {
            var result = new long[array.Length];
            for (var i = 0; i < array.Length; i++)
            {
                result[i] = long.Parse(array[i]);
            }
            return result;
        }
        private static double[] ToDoubleArray(string[] array)
        {
            var result = new double[array.Length];
            for (var i = 0; i < array.Length; i++)
            {
                result[i] = double.Parse(array[i]);
            }
            return result;
        }
        #endregion
    }
    #endregion
}


Measured memory_bytes: 17232