import java.util.Scanner;
class Main{
    public static void main(String arg[]){
        Scanner scn=new Scanner(System.in);
        int n=scn.nextInt();
        int m=scn.nextInt();
        int L=scn.nextInt();
        int R=scn.nextInt();
        for(int i=0;i<2*(m-1);i++){
            int buf=scn.nextInt();
            if(i%2==0){
                if(L<buf){
                    L = buf;
                }
            }else if(i%2==1){
                if(buf<R){
                    R=buf;
                }
            }
        }    
            
         if(L>R){
            System.out.println(0);
        }else{
            System.out.println(R-L+1);
        }
    }
}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz