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.math.BigInteger;
import java.text.DecimalFormat;
import java.util.*;

public class Newbie {

    static InputReader sc = new InputReader(System.in);
    static PrintWriter out = new PrintWriter(System.out);

    public static void main(String[] args) throws IOException {
        solver s = new solver();
        long t = 1;
        while (t > 0) {
            s.solve();
            t--;
        }
        out.close();
    }

   /* static class descend implements Comparator<pair1> {
        public int compare(pair1 o1, pair1 o2) {
            if (o1.pop != o2.pop)
                return (int) (o1.pop - o2.pop);
            else
                return o1.in - o2.in;
        }
    }*/

    static class InputReader {
        public BufferedReader br;
        public StringTokenizer token;

        public InputReader(InputStream stream) {
            br = new BufferedReader(new InputStreamReader(stream), 32768);
            token = null;
        }

        public String sn() {
            while (token == null || !token.hasMoreTokens()) {
                try {
                    token = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return token.nextToken();
        }

        public int ni() {
            return Integer.parseInt(sn());
        }

        public String snl() throws IOException {
            return br.readLine();
        }

        public long nlo() {
            return Long.parseLong(sn());
        }

        public double nd() {
            return Double.parseDouble(sn());
        }

        public int[] na(int n) {
            int a[] = new int[n];
            for (int i = 0; i < n; i++)
                a[i] = sc.ni();
            return a;
        }

        public long[] nal(int n) {
            long a[] = new long[n];
            for (int i = 0; i < n; i++)
                a[i] = sc.nlo();
            return a;
        }
    }

    static class card {
        long a;
        int cnt;
        int i;

        public card(long a, int cnt, int i) {
            this.a = a;
            this.cnt = cnt;
            this.i = i;
        }
    }

    static class ascend implements Comparator<pair> {
        public int compare(pair o1, pair o2) {
            if (o1.a != o2.a)
                return (int) (o1.a - o2.a);
            else
                return o1.b - o2.b;
        }
    }

    static class extra {

        static boolean v[] = new boolean[100001];
        static List<Integer> l = new ArrayList<>();
        static int t;

        static void shuffle(int a[]) {
            for (int i = 0; i < a.length; i++) {
                int t = (int) Math.random() * a.length;
                int x = a[t];
                a[t] = a[i];
                a[i] = x;
            }
        }

        static void shufflel(long a[]) {
            for (int i = 0; i < a.length; i++) {
                int t = (int) Math.random() * a.length;
                long x = a[t];
                a[t] = a[i];
                a[i] = x;
            }
        }

        static long gcd(long a, long b) {
            if (b == 0)
                return a;
            else
                return gcd(b, a % b);
        }

        static boolean valid(int i, int j, int r, int c, char ch[][]) {
            if (i >= 0 && i < r && j >= 0 && j < c && ch[i][j] != '*') {
                // System.out.println(i+" /// "+j);
                return true;
            } else {
                //  System.out.println(i+" //f "+j);
                return false;
            }
        }

        static void seive() {
            for (int i = 2; i < 100001; i++) {
                if (!v[i]) {
                    t++;
                    l.add(i);
                    for (int j = 2 * i; j < 100001; j += i)
                        v[j] = true;
                }
            }
        }

        static int binary(LinkedList<Integer> a, long val, int n) {
            int mid = 0, l = 0, r = n - 1, ans = 0;
            while (l <= r) {
                mid = (l + r) >> 1;
                if (a.get(mid) == val) {
                    r = mid - 1;
                    ans = mid;
                } else if (a.get(mid) > val)
                    r = mid - 1;
                else {
                    l = mid + 1;
                    ans = l;
                }
            }
            return (ans + 1);
        }

        static long fastexpo(long x, long y) {
            long res = 1;
            while (y > 0) {
                if ((y & 1) == 1) {
                    res *= x;
                }
                y = y >> 1;
                x = x * x;
            }
            return res;
        }

        static long lfastexpo(long x, long y, long p) {
            long res = 1;
            x = x % p;
            while (y > 0) {
                if ((y & 1) == 1) {
                    res = (res * x) % p;
                }
                y = y >> 1;
                x = (x * x) % p;
            }
            return res;
        }

       /* void dijsktra(int s, List<pair> l[], int n) {
            PriorityQueue<pair> pq = new PriorityQueue<>(new ascend());
            int dist[] = new int[100005];
            boolean v[] = new boolean[100005];
            for (int i = 1; i <= n; i++)
                dist[i] = 1000000000;
            dist[s] = 0;
            for (int i = 1; i < n; i++) {
                if (i == s)
                    pq.add(new pair(s, 0));
                else
                    pq.add(new pair(i, 1000000000));
            }
            while (!pq.isEmpty()) {
                pair node = pq.remove();
                v[node.a] = true;
                for (int i = 0; i < l[node.a].size(); i++) {
                    int v1 = l[node.a].get(i).a;
                    int w = l[node.a].get(i).b;
                    if (v[v1])
                        continue;
                    if ((dist[node.a] + w) < dist[v1]) {
                        dist[v1] = dist[node.a] + w;
                        pq.add(new pair(v1, dist[v1]));
                    }
                }
            }
        }*/
    }

    static class pair {
        long a;
        int b;

        public pair(long a, int i) {
            this.a = a;
            this.b = i;
        }
    }

    static class pair1 {
        pair p;
        int in;

        public pair1(pair a, int n) {
            this.p = a;
            this.in = n;
        }
    }


    static int inf = 5000013;

    static class solver {
        DecimalFormat df = new DecimalFormat("0.000000");
        extra e = new extra();
        int mod = 1000000007;

        void solve() {
            int n = sc.ni();
            int m = sc.ni();
            int max = 1;
            int min = n;
            for (int i = 0; i < m; i++) {
                int a = sc.ni();
                int b = sc.ni();
                if (a > b) {
                    int t = a;
                    a = b;
                    b = t;
                }
                max=Math.max(a,max);
                min=Math.min(b,min);
            }
            out.println(Math.max(0,min-max));
        }
    }
}


