(***
      PICTURE - PASCAL version 1
      ps@di.fct.unl.pt
 ***)

program Picture (output);

	const
		maxRect = 5000;
		maxEdges = 10000;   { 2*maxRect }
		XX = false;
		YY = true;

	type
		RectList = array[1..maxRect] of boolean;

		Items = record
				value: integer;
				first: boolean;
				nR: 1..maxRect
			end;

		EdgeList = array[boolean, 1..maxEdges] of Items;

	var
		f, g: text;
		Edge: EdgeList;
		MarkRect: RectList;
		NRect: integer;
		Perimeter: longint;

	procedure Abort (n: integer);
	begin
		case n of
			1:
				writeln('*** Empty File ***');
			2:
				writeln('*** Too many pictures ***')
		end;
		Halt
	end;

	procedure ReadFile;
		var
			i: integer;
			axes: boolean;
	begin
		if eof(f) then
			Abort(1);
		readln(f, NRect);
		if NRect > maxRect then
			Abort(2);
		for i := 1 to NRect do
			begin
				MarkRect[i] := false;
				read(f, Edge[XX, 2 * i - 1].value, Edge[YY,
2 * i - 1].value);
				readln(f, Edge[XX, 2 * i].value, Edge[YY, 2
* i].value);
				for axes := XX to YY do
					begin
						Edge[axes, 2 * i - 1].first
:= true;
						Edge[axes, 2 * i - 1].nR := i;
						Edge[axes, 2 * i].first :=
false;
						Edge[axes, 2 * i].nR := i
					end
			end;
	end;

	procedure SortEdges (axes: boolean);
		var
			aux: Items;
			swap: boolean;
			i, j: integer;
	begin
		swap := true;
		i := 1;
		j := 0;
		while swap do
			begin
				swap := false;
				for i := 2 to 2 * NRect - j do
					if Edge[axes, i].value < Edge[axes,
i - 1].value then
						begin
							aux := Edge[axes, i
- 1];
							Edge[axes, i - 1]
:= Edge[axes, i];
							Edge[axes, i] := aux;
							swap := true
						end;
				j := j + 1
			end
	end;

	procedure ScanRect (axes: boolean);  { only if NRect>0 }
		var
			i, scanLine, nextScanLine: integer;

		procedure ScanBetween (axes: boolean; sline, nsline: integer);
			var
				i, TotalRect, OldTotalRect, previous: integer;
				start: boolean;
		begin
			TotalRect := 0;
			OldTotalRect := 0;
			start := true;
			for i := 1 to NRect * 2 do
				if MarkRect[Edge[axes, i].nR] then
					begin
						if Edge[axes, i].first then
							TotalRect :=
TotalRect + 1
						else
							TotalRect :=
TotalRect - 1;
						if (OldTotalRect = 1) and
(TotalRect = 0) then
							Perimeter :=
Perimeter + (nsline - sline)
						else if (OldTotalRect = 0)
and (TotalRect = 1) then
							if start then
								Perimeter
:= Perimeter + (nsline - sline)
							else if Edge[axes,
i].value = previous then
								Perimeter
:= Perimeter - (nsline - sline)
							else
								Perimeter
:= Perimeter + (nsline - sline);
						previous := Edge[axes,
i].value;
						start := false;
						OldTotalRect := TotalRect
					end
		end;

	begin
		i := 1;
		nextScanLine := Edge[axes, i].value;
		while i < NRect * 2 do
			begin
				scanLine := nextScanLine;
				MarkRect[Edge[axes, i].nR] := Edge[axes,
i].first;
				while Edge[axes, i].value = Edge[axes, i +
1].value do
					begin
						i := i + 1;
						MarkRect[Edge[axes, i].nR]
:= Edge[axes, i].first
					end;
				nextScanLine := Edge[axes, i + 1].value;
				if nextScanLine > scanLine then
					ScanBetween(not axes, scanLine,
nextScanLine);
				i := i + 1
			end
	end;

	procedure ClearMarks;
		var
			i: integer;
	begin
		for i := 1 to NRect do
			MarkRect[i] := false
	end;

begin (* MAIN *)
	assign(f, 'PICTURE.IN');
	reset(f);
	ReadFile;
        close(f);
	Perimeter := 0;
	if NRect > 0 then
		begin
			SortEdges(XX);
			SortEdges(YY);
			ScanRect(XX);
			ClearMarks;
			ScanRect(YY)
		end;
	assign(g, 'PICTURE.OUT');
	rewrite(g);
	writeln(g, Perimeter : 1);
        close(g);
	writeln;
	writeln('Perimeter=', Perimeter : 1)
end.