import java.util.Scanner;
public class Main {
public static void main(String[] args) {
int l,m;
Scanner scanner=new Scanner(System.in);
l=scanner.nextInt();
m=scanner.nextInt();
boolean[] a=new boolean[l+1];
while(m-->0){
int st,end;
st=scanner.nextInt();
end=scanner.nextInt();
for(int i=st;i<=end;i++){
a[i]=true;
}
}
int res=0;
for(int i=0;i<=l;i++){
if(a[i]==false) res++;
}
System.out.println(res);
}
}