java

import java.util.*;
class Plant_trees {
    public static void main (String args[]){
        Scanner sc = new Scanner(System.in);
        int a,b;
        a = sc.nextInt();
        b = sc.nextInt();
        int []start = new int[b];
        int []end = new int[b];
        for(int i = 0;i<b;i++){
            start[i] = sc.nextInt();
            end[i] = sc.nextInt();
        }
        int []tree = new int[a+1];
        for(int i=0;i < a+1;i++){
            tree[i] = 1;
        }
        for(int i = 0;i < b;i++){
            for(int j = start[i];j <= end[i];j++){
                if(tree[j]==1){
                    tree[j] = 0;
                }
            }
        }
        int count = 0;
        for(int i = 0;i < a+1;i++){
            if(tree[i]==1){
                count++;
            }
        }
        System.out.println(count);
    }
}