var v:array[1..32000] of integer;
j,i,nr,n,a,b,max:integer;
f,g:text;
begin
assign(f,'heavymetal.in');reset(f);
assign(g,'heavymetal.out');rewrite(g);
readln(f,n);
max:=1;
nr:=0;
for i:=1 to n do
begin
read(f,a);
readln(f,b);
if b>max then max:=b;
for j:=a to b-1 do inc(v[j]);
end;
for i:=1 to max do
if v[i]<>0 then inc(nr);
write(g,nr);
close(f);
close(g);
end.