C++ :

#include<iostream>
#include<cstring>

using namespace std;
int main()
{
	int l,m,a,b,c=0;
	cin>>l>>m;
	bool s[10001];
	memset(s,1,sizeof(s));
	for(int i=1;i<=m;i++)
	{
	  cin>>a>>b;
	  for(int j=a;j<=b;j++)
	      s[j]=0;
    }
    for(int i=0;i<=l;i++)
     if(s[i]==1)
	  c++;
	  cout<<c;
}

Pascal :

type
 rec=record
  x,y,l,r,c:longint;
 end;
var
 ans,l,m,i,x,y,tot:longint;
 tree:array[0..40000] of rec;
procedure make(l,r:longint);
var
 now:longint;
begin
 inc(tot);
 now:=tot;
 tree[now].x:=l;
 tree[now].y:=r;
 if l+1<=r then
 begin
  tree[now].l:=tot+1;
  make(l,(l+r) div 2);
  tree[now].r:=tot+1;
  make((l+r) div 2+1,r);
 end;
end;
procedure insert(v,x,y:longint);
begin
 if (x<=tree[v].x)and(tree[v].y<=y) then
 begin
  inc(tree[v].c);
  exit;
 end;
 if x<=(tree[v].x+tree[v].y) div 2 then insert(tree[v].l,x,y);
 if y>(tree[v].x+tree[v].y) div 2 then insert(tree[v].r,x,y);
end;
procedure find(v:longint);
begin
 if v=0 then exit;
 if tree[v].c>0 then
 begin
  inc(ans,tree[v].y-tree[v].x+1);
  exit;
 end;
 find(tree[v].l);
 find(tree[v].r);
end;
begin
 readln(l,m);
 make(1,l+1);
 for i:=1 to m do
 begin
  readln(x,y);
  insert(1,x+1,y+1);
 end;
 find(1);
 //writeln(ans,' ',l);
 writeln(l+1-ans);
end.