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

namespace ProgrammingContest
{
    class Writer : IDisposable
    {
        private readonly System.IO.TextWriter writer;
        private StringBuilder sb;
        private readonly bool isReactive;
        public Writer(string path)
            : this(new System.IO.StreamWriter(path))
        {
        }
        public Writer(bool isReactive)
            : this(null, isReactive)
        {
        }
        public Writer(System.IO.TextWriter writer = null,
                        bool isReactive = false)
        {
            this.writer = (writer ?? Console.Out);
            this.isReactive = isReactive;
            if (!this.isReactive)
            {
                this.sb = new StringBuilder();
            }
        }
        public void Dispose()
        {
            if (!this.isReactive)
            {
                this.writer.Write(sb.ToString());
            }
            if (!this.writer.Equals(Console.Out))
            {
                this.writer.Dispose();
            }
        }
        public void Write(object val)
        {
            if (this.isReactive)
            {
                this.writer.Write(val.ToString());
                this.writer.Flush();
            }
            else
            {
                this.sb.Append(val.ToString());
            }
        }
        public void Write(string format, params object[] vals)
        {
            if (this.isReactive)
            {
                this.writer.Write(format, vals);
                this.writer.Flush();
            }
            else
            {
                this.sb.AppendFormat(format, vals);
            }
        }
        public void WriteLine(object val = null)
        {
            this.Write((val ?? string.Empty).ToString() + System.Environment.NewLine);
        }
        public void WriteLine(int val)
        {
            this.WriteLine(val.ToString());
        }
        public void WriteLine(long val)
        {
            this.WriteLine(val.ToString());
        }
        public void WriteLine(string val)
        {
            this.WriteLine((object)val);
        }
        public void WriteLine(string format, params object[] vals)
        {
            this.Write(format + System.Environment.NewLine, vals);
        }
    }

    class Scanner : IDisposable
    {
        private readonly Queue<string> buffer;
        private readonly char[] sep;
        private readonly System.IO.TextReader reader;
        public Scanner(string path, char[] sep = null)
            : this(new System.IO.StreamReader(path), sep)
        {
        }
        public Scanner(System.IO.TextReader reader = null,
                        char[] sep = null)
        {
            this.buffer = new Queue<string>();
            this.sep = (sep ?? new char[] { ' ' });
            this.reader = (reader ?? Console.In);
        }
        private void CheckBuffer()
        {
            if (this.buffer.Count == 0 && this.reader.Peek() != -1)
            {
                string str = string.Empty;
                for (; string.IsNullOrEmpty(str);
                    str = this.reader.ReadLine()) ;

                str.Split(this.sep).ToList()
                   .ForEach(el => this.buffer.Enqueue(el));
            }
        }

        public void Dispose()
        {
            if (!this.reader.Equals(Console.In))
            {
                this.reader.Dispose();
            }
        }

        public string Next
        {
            get
            {
                this.CheckBuffer();
                return this.buffer.Dequeue();
            }
        }
        public string[] GetStringArray(int N)
        {
            return Enumerable.Range(0, N)
                             .Select(e => this.Next)
                             .ToArray();
        }

        public int NextInt
        {
            get
            {
                return int.Parse(this.Next);
            }
        }
        public int[] GetIntArray(int N)
        {
            return Enumerable.Range(0, N)
                             .Select(e => this.NextInt)
                             .ToArray();
        }

        public double NextDouble
        {
            get
            {
                return double.Parse(this.Next);
            }
        }
        public double[] GetdoubleArray(int N)
        {
            return Enumerable.Range(0, N)
                             .Select(e => this.NextDouble)
                             .ToArray();
        }

        public long NextLong
        {
            get
            {
                return long.Parse(this.Next);
            }
        }
        public long[] GetLongArray(int N)
        {
            return Enumerable.Range(0, N)
                             .Select(e => this.NextLong)
                             .ToArray();
        }

        public bool IsEnd
        {
            get
            {
                this.CheckBuffer();
                return this.buffer.Count == 0;
            }
        }
    }

    class MainClass : IDisposable
    {
        Scanner sc;
        Writer wr;
        const string backPath = "..";
        private readonly char dirSep = System.IO.Path.DirectorySeparatorChar;
        private readonly string inFilePath = string.Empty;
        private readonly string outFilePath = string.Empty;
        public MainClass()
        {
            this.inFilePath = MainClass.backPath
                            + this.dirSep
                            + MainClass.backPath
                            + this.dirSep
                            + "in.txt";
            this.outFilePath = MainClass.backPath
                             + this.dirSep
                             + MainClass.backPath
                             + this.dirSep
                             + "out.txt";

            this.wr = new Writer(this.IsReactive);
            //this.wr = new Writer(this.outFilePath);
#if DEBUG
            if (!this.IsReactive)
            {
                this.sc = new Scanner(this.inFilePath);
            }
            else
            {
                this.sc = new Scanner();
            }
#else
            this.sc = new Scanner();
#endif
        }
        static void Main(string[] args)
        {
            using (var mainClass = new MainClass())
            {
                mainClass.Solve();
            }
        }

        public void Dispose()
        {
            if (this.sc != null)
            {
                this.sc.Dispose();
                this.sc = null;
            }
            if (this.wr != null)
            {
                this.wr.Dispose();
                this.wr = null;
            }
#if DEBUG
            Console.WriteLine("press any key to continue...");
            Console.ReadKey();
#endif
        }

        void MakeTestCase()
        {
            var rand = new Random();
            if (this.wr != null)
            {
                this.wr.Dispose();
            }
            this.wr = new Writer(inFilePath);
        }

        void Solve()
        {
            int N = sc.NextInt;
            int M = sc.NextInt;
            var ab = Enumerable.Range(0, M).Select(e => new Pair<int, int>(sc.NextInt - 1, sc.NextInt - 1));
            var se = Enumerable.Range(0, N).Select(e => new HashSet<int>()).ToArray();
            foreach (var p in ab)
            {
                se[p.First].Add(p.Second);
                se[p.Second].Add(p.First);
            }
            UnionFind uf = new UnionFind(N);
            int cnt = 0;
            for (int i = 0; i < N - 1; i++)
            {
                bool ok = true;
                foreach (var e in se[i + 1])
                {
                    if (uf.Same(i, e))
                    {
                        ok = false;
                        break;
                    }
                }
                if (ok)
                {
                    uf.Unite(i, i + 1);
                    cnt++;
                }
            }
            wr.WriteLine(N - 1 - cnt);
        }

        class UnionFind
        {
            private int[] uni;
            int count_;
            public UnionFind(int n)
            {
                this.uni = new int[n];
                for (int i = 0; i < n; i++)
                {
                    this.uni[i] = -1;
                }
                this.count_ = n;
            }
            public int Count()
            {
                return this.count_;
            }
            public int Count(int n)
            {
                return -this.uni[this.Find(n)];
            }
            public int Find(int n)
            {
                return (this.uni[n] < 0 ? n : this.uni[n] = this.Find(this.uni[n]));
            }
            public bool Unite(int a, int b)
            {
                a = this.Find(a);
                b = this.Find(b);
                if (a == b)
                {
                    return false;
                }
                if (this.uni[a] > this.uni[b])
                {
                    int t = a;
                    a = b;
                    b = t;
                }
                this.uni[a] += this.uni[b];
                this.uni[b] = a;
                this.count_--;
                return true;
            }
            public bool Same(int a, int b)
            {
                return this.Find(a) == this.Find(b);
            }
        }

        class Pair<T1, T2> : IComparable<Pair<T1, T2>>
        {
            public T1 First { get; set; }
            public T2 Second { get; set; }

            public Pair() { }

            public Pair(T1 f, T2 s)
            {
                this.First = f;
                this.Second = s;
            }

            public int CompareTo(Pair<T1, T2> other)
            {
                int ret = 0;
                if (First is IComparable<T1>)
                {
                    ret = (First as IComparable<T1>).CompareTo(other.First);
                }
                if (ret == 0 && Second is IComparable<T2>)
                {
                    ret = (Second as IComparable<T2>).CompareTo(other.Second);
                }
                return ret;

            }
        }

        bool NextPermutation<T>(T[] ar) where T : IComparable<T>
        {
            int left = ar.Length - 1;
            Func<int, int, int> CompForCalcNextPermutation = (i, j) =>
            {
                return ar[i].CompareTo(ar[j]);
            };
            while (0 < left && CompForCalcNextPermutation(left - 1, left) >= 0)
            {
                left--;
            }
            if (left == 0)
            {
                return false;
            }
            int right = ar.Length - 1;
            while (CompForCalcNextPermutation(right, left - 1) <= 0)
            {
                right--;
            }
            T tmp = ar[left - 1];
            ar[left - 1] = ar[right];
            ar[right] = tmp;
            Array.Reverse(ar, left, ar.Length - left);
            return true;
        }

        private bool IsReactive { get; } = false; // TODO: reactive check !!
    }
}


Measured memory_bytes: 40972