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 Java8 (OpenJDK 1.8.0) (judge verdict: Runtime Error)

import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.io.BufferedWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.Comparator;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author L_aka_ryuga
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        CPrison solver = new CPrison();
        solver.solve(1, in, out);
        out.close();
    }

    static class CPrison {
        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int n = in.ni(), m = in.ni();
            gate[] object = new gate[m];

            for (int i = 0; i < m; ++i) {
                int l = in.ni(), r = in.ni();
                object[i] = new gate(l, r);
            }
            checker check = new checker();
            Arrays.sort(object, check);

       /* for(int i = 0;i < m;++i){
            out.println(object[i].val1+"  "+object[i].val2);
        }
*/

        /*
        int[] arr = new int[100003];

        for(int i = 0;i < m;++i){

            arr[l] = r;

        }
        */
            int[][] gates = new int[m][2];
            int k = 0;
            for (int i = 0; i < m; ++i) {
                gates[k][0] = object[i].val1;
                gates[k++][1] = object[i].val2;
            }

            int a = gates[0][0], b = gates[0][1];
//        out.println(a+" "+b);
            int val = b - a + 1, ans = val;
            for (int i = 1; i < m; ++i) {
                int c = gates[i][0], d = gates[i][1];
//            out.println(c+"  "+d);
                if (c > b) ans = 0;
                else {
                    if (d >= b) ans = (b - c + 1);
                    else ans = (d - c + 1);
                }
                val = Math.min(val, ans);
            }

            out.println(val);

        }

    }

    static class checker implements Comparator {
        public int compare(Object o, Object t1) {
            gate ob = (gate) o;
            gate ob2 = (gate) t1;
            int v = (int) ob.val1, t = (int) ob2.val1;
            if (v <= t)
                return -1;
            else return 1;
        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void close() {
            writer.close();
        }

        public void println(int i) {
            writer.println(i);
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.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 ni() {
            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 boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

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

        }

    }

    static class gate {
        int val1;
        int val2 = 0;

        gate(int val1, int val2) {
            this.val1 = val1;
            this.val2 = val2;
        }

    }
}