const
  maxnum=10000000;
var
  n,m,i,j,vle,vri,vans:longint;
  data:array[0..maxnum] of integer;
  dause:array[0..1000] of boolean;

begin
  assign(input,'ha14d.in');
  reset(input);
  assign(output,'ha14d.out');
  rewrite(output);
  readln(n,m);
  fillchar(dause,sizeof(dause),false);
  for i:=1 to m do
    begin
      readln(vle,vri);
      for j:=vle to vri do
          data[j]:=i;
      dause[i]:=true;
    end;

  vans:=0;
  for i:=1 to n do
    begin
      if (dause[data[i]]) then
        inc(vans);
      dause[i]:=false;
    end;

  writeln(vans);
  close(input);
  close(output);
end.