There are n problems prepared for the next Codeforces round. They are arranged in ascending order by their difficulty, and no two problems have the same difficulty. Moreover, there are m pairs of similar problems. Authors want to split problems between two division according to the following rules: 

  * Problemset of each division should be non-empty. 
  * Each problem should be used in exactly one division (yes, it is unusual requirement). 
  * Each problem used in division 1 should be harder than any problem used in division 2. 
  * If two problems are similar, they should be used in different divisions. 



Your goal is count the number of ways to split problem between two divisions and satisfy all the rules. Two ways to split problems are considered to be different if there is at least one problem that belongs to division 1 in one of them and to division 2 in the other.

Note, that the relation of similarity is not transitive. That is, if problem i is similar to problem j and problem j is similar to problem k, it doesn't follow that i is similar to k.

Input

The first line of the input contains two integers n and m (2 ≤ n ≤ 100 000, 0 ≤ m ≤ 100 000) — the number of problems prepared for the round and the number of pairs of similar problems, respectively.

Each of the following m lines contains a pair of similar problems ui and vi (1 ≤ ui, vi ≤ n, ui ≠ vi). It's guaranteed, that no pair of problems meets twice in the input.

Output

Print one integer — the number of ways to split problems in two divisions.

Examples

Input

5 2
1 4
5 2


Output

2


Input

3 3
1 2
2 3
1 3


Output

0


Input

3 2
3 1
3 2


Output

1

Note

In the first sample, problems 1 and 2 should be used in division 2, while problems 4 and 5 in division 1. Problem 3 may be used either in division 1 or in division 2.

In the second sample, all pairs of problems are similar and there is no way to split problem between two divisions without breaking any rules.

Third sample reminds you that the similarity relation is not transitive. Problem 3 is similar to both 1 and 2, but 1 is not similar to 2, so they may be used together.


import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;
import java.lang.*;


public class B{
	static long mod=1000000007;
	static boolean v[];
	static void dfs(int i)
	{
		
	}
    public static void main(String[] args) throws Exception{           	
    	 InputReader in = new InputReader(System.in);       
    	PrintWriter pw=new PrintWriter(System.out);   
    
    	//int t=in.readInt();
        //	while(t-->0)
        	//{
        	int n=in.readInt();
         	int m=in.readInt();
    	//long n=in.readLong();
    	int a[]=new int[n];
    	ArrayList<Integer>l=new ArrayList<Integer>();
    	ArrayList<Integer>r=new ArrayList<Integer>();
    	HashSet<Integer>hs1=new HashSet<Integer>();
    	HashSet<Integer>hs2=new HashSet<Integer>();
    	int ans=n-1;
    	int g=1;
    	for(int i=0;i<m;i++)
    	{
    		
    		int x=in.readInt();
    		int y=in.readInt();
    		x--;
    		y--;
    		if(x<y)
    		{
    			int t=x;
    			x=y;
    			y=t;
    		}
    		if(hs2.contains(x))
    				{
    				g=0;
    				ans=0;
    				}
    		if(hs1.contains(y))
			{
			g=0;
			ans=0;
			}
    		l.add(x);
    		hs1.add(x);
    		hs2.add(y);
    		r.add(y);
    	}
    	
    		Collections.sort(l);
    		Collections.sort(r);
    		if(m==0||g==0)
    		{
    			 		
    		}
    		else if(l.size()>1)
    		{
    				int x=r.get(r.size()-1);
        			int y=l.get(0);
        			if(y>x)
        				ans=y-x;
        			else
        				ans=0;
    			
    		}
    		else
    		{
    			int x=r.get(r.size()-1);
    			int y=l.get(0);
    			ans=y-x;
    		}
    		pw.println(ans);   	   
    	pw.close();
    }
    
   
  
    /* returns nCr mod m */
   	public static long comb(long n, long r, long m) 
   	{
   		long p1 = 1, p2 = 1;
   		for (long i = r + 1; i <= n; i++) {
   			p1 = (p1 * i) % m;
   		}
   		p2 = factMod(n - r, m);
   		p1 = divMod(p1, p2, m);
   		return p1 % m;
   	}
   /* returns a/b mod m, works only if m is prime and b divides a */
   	public static long divMod(long a, long b, long m) 
   	{
   		long c = powerMod(b, m - 2, m);
   		return ((a % m) * (c % m)) % m;
   	}
   /* calculates factorial(n) mod m */
   	public static long factMod(long n, long m) {
   		long result = 1;
   		if (n <= 1)
   			return 1;
   		while (n != 1) {
   			result = ((result * n--) % m);
   		}
   		return result;
   	}
   /* This method takes a, b and c as inputs and returns (a ^ b) mod c */
   	public static long powerMod(long a, long b, long c) {
   		long result = 1;
   		long temp = 1;
   		long mask = 1;
   		for (int i = 0; i < 64; i++) {
   			mask = (i == 0) ? 1 : (mask * 2);
   			temp = (i == 0) ? (a % c) : (temp * temp) % c;
   			/* Check if (i + 1)th bit of power b is set */
   			if ((b & mask) == mask) {
   				result = (result * temp) % c;
   			}
   		}
   		return result;
   	}
	
	
public static long gcd(long x,long y)
{
	if(x%y==0)
		return y;
	else
		return gcd(y,x%y);
}
	
public static int gcd(int x,int y)
{
	if(x%y==0)
		return y;
	else 
		return gcd(y,x%y);
}
public static int abs(int a,int b)
{
	return (int)Math.abs(a-b);
}
public static long abs(long a,long b)
{
	return (long)Math.abs(a-b);
}
public static int max(int a,int b)
{
	if(a>b)
		return a;
	else
		return b;
}
public static int min(int a,int b)
{
	if(a>b)
		return b;
	else 
		return a;
}
public static long max(long a,long b)
{
	if(a>b)
		return a;
	else
		return b;
}
public static long min(long a,long b)
{
	if(a>b)
		return b;
	else 
		return a;
}

static boolean isPowerOfTwo (long v) {
	return (v & (v - 1)) == 0;
}
public static long pow(long n,long p,long m)
{
	 long  result = 1;
	  if(p==0)
	    return 1;
	if (p==1)
	    return n;
	while(p!=0)
	{
	    if(p%2==1)
	        result *= n;
	    if(result>=m)
	    result%=m;
	    p >>=1;
	    n*=n;
	    if(n>=m)
	    n%=m;
	}
	return result;
}
public static long pow(long n,long p)
{
	long  result = 1;
	  if(p==0)
	    return 1;
	if (p==1)
	    return n;
	while(p!=0)
	{
	    if(p%2==1)
	        result *= n;	    
	    p >>=1;
	    n*=n;	    
	}
	return result;

}
static class Pair implements Comparable<Pair>
{
	int a,b;
	Pair (int a,int b)
	{
		this.a=a;
		this.b=b;
	}

	public int compareTo(Pair o) {
		// TODO Auto-generated method stub
		if(this.a!=o.a)
		return Integer.compare(this.a,o.a);
		else
			return Integer.compare(this.b, o.b);
		//return 0;
	}
	public boolean equals(Object o) {
        if (o instanceof Pair) {
            Pair p = (Pair)o;
            return p.a == a && p.b == b;
        }
        return false;
    }
    public int hashCode() {
        return new Integer(a).hashCode() * 31 + new Integer(b).hashCode();
    }
} 
    
static long sort(int a[],int n)
{  	int b[]=new int[n];	
	return mergeSort(a,b,0,n-1);}
static long mergeSort(int a[],int b[],long left,long right)
{ long c=0;if(left<right)
 {   long mid=left+(right-left)/2;
	 c= mergeSort(a,b,left,mid);
	 c+=mergeSort(a,b,mid+1,right);
	 c+=merge(a,b,left,mid+1,right); }	
	return c;	 }
static long merge(int a[],int  b[],long left,long mid,long right)
{long c=0;int i=(int)left;int j=(int)mid; int k=(int)left;
while(i<=(int)mid-1&&j<=(int)right)
{ if(a[i]<=a[j]) {b[k++]=a[i++]; }
else	{ b[k++]=a[j++];c+=mid-i;}}
while (i <= (int)mid - 1)   b[k++] = a[i++]; 
while (j <= (int)right) b[k++] = a[j++];
for (i=(int)left; i <= (int)right; i++) 
	a[i] = b[i];  return c;  }
    
    
   static  class InputReader
    {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream)
        {
            this.stream = stream;
        }

        public int read()
        {
            if (numChars == -1)
                throw new InputMismatchException();
            if (curChar >= numChars)
            {
                curChar = 0;
                try
                {
                    numChars = stream.read(buf);
                } catch (IOException e)
                {
                    throw new InputMismatchException();
                }
                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int readInt()
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-')
            {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do
            {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public String readString()
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do
            {
                res.appendCodePoint(c);
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }
        public double readDouble() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            double res = 0;
            while (!isSpaceChar(c) && c != '.') {
                if (c == 'e' || c == 'E')
                    return res * Math.pow(10, readInt());
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            if (c == '.') {
                c = read();
                double m = 1;
                while (!isSpaceChar(c)) {
                    if (c == 'e' || c == 'E')
                        return res * Math.pow(10, readInt());
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    m /= 10;
                    res += (c - '0') * m;
                    c = read();
                }
            }
            return res * sgn;
        }
        public long readLong() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }
        public boolean isSpaceChar(int c)
        {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public String next()
        {
            return readString();
        }

        public interface SpaceCharFilter
        {
            public boolean isSpaceChar(int ch);
        }
    }

  
//BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
	//StringBuilder sb=new StringBuilder("");
	 //InputReader in = new InputReader(System.in);
	//PrintWriter pw=new PrintWriter(System.out);
	//String line=br.readLine().trim();
	       	
	//int t=Integer.parseInt(br.readLine());
  //	while(t-->0)
  	//{
  	//int n=Integer.parseInt(br.readLine());
	//long n=Long.parseLong(br.readLine());
	//String l[]=br.readLine().split(" ");
  //int m=Integer.parseInt(l[0]);
	//int k=Integer.parseInt(l[1]);
	//String l[]=br.readLine().split(" ");
	//l=br.readLine().split(" ");
	/*int a[]=new int[n];
	for(int i=0;i<n;i++)
	{
		a[i]=Integer.parseInt(l[i]);
	}*/
	   //System.out.println(" ");	     	
	
	//}
}