const Max = 1000;
type List = array[1..Max] of Integer;
var v1,v2:list;
iny,aux,n,i,j:integer;
f:text;suma:longint;
procedure citire;
var i:integer;
begin
assign(f,'heavymetal.in');reset(f);
readln(f,n);
for i:=1 to n do readln(f,v1[i],v2[i]);
close(f);
end;
begin
citire;
for i:=1 to n-1 do
for j:=i+1 to n do
if v1[i]>v2[j] then
begin
aux:=v1[i];
v1[i]:=v1[j];
v1[j]:=aux;
aux:=v2[i];
v2[i]:=v2[j];
v2[j]:=aux;
end;
for i:=1 to n-1 do
begin
if v1[i]=v1[i+1] then
begin
if v2[i]>v2[i+1] then
begin
v1[i+1]:=0; v2[i+1]:=0;
end
else
v1[i]:=0; v2[i]:=0;
end;
end;
suma:=0;
suma:=v2[1]-v1[1];
iny:=1;
for i:=2 to n do
if v1[i]<>0 then
begin
if v1[i]>=v2[iny] then
begin
suma:=suma+(v2[i]-v1[i]);
inc(iny);
end;
end;
assign(f,'heavymetal.out');rewrite(f);
writeln(f,suma);
close(f);
end.