#include<iostream>
using namespace std;
struct Note{
int begin;
int end;
Note *next;
};
int main()
{
int l,m;
int i;
Note *p,*q;
cin >> l >> m;
Note *head = new Note;
Note *tail;
head->next = NULL;
cin >> head->begin >> head->end;
tail = head;
for(i=1;i<m;i++)
{//依次讀入m個區域，並按照升序排列區域
p = new Note;
cin >> p->begin >> p->end ;
p->next = NULL;
q = head;
if( p->end < head->begin )
{//在鏈表頭插入節點
p->next = head;
head = p;
continue;
}
if( p->begin > tail->end )
{//在鏈表尾插入節點
tail->next = p;
tail = p;
continue;
}
while(q)
{
if( p->begin<=q->begin && p->end>=q->begin || p->begin<=q->end && p->end<q->next->begin )
{//兩個區域相交
q->begin = p->begin<q->begin ? p->begin : q->begin;
q->end = p->end < q->end ? q->end : p->end;
delete p;
break;
}
else if( p->begin<=q->end && p->end>=q->next->begin )
{//新增區域連接兩個相鄰區域
q->begin = p->begin<q->begin ? p->begin : q->begin;
q->end = q->next->end;
delete p;
q->next = q->next->next;
p = q->next;
delete p;
break;
}
else if( p->begin>q->end && p->end<q->next->begin )
{//在某個節點後插入新增區域
p->next = q->next;
q->next = p;
break;
}
q = q->next;
}
}
l++; //從0到l有l+1棵樹
q = head;
while(q)
{
//cout << q->begin << '\t' << q->end << endl;調試用
l -= q->end - q->begin + 1;
q = q->next;
}
cout << l << endl;
return 0;
}