ИНФОРМАТИКА 6 ЗАДАЧ ПОМОГИТЕ

var
x, y: longint;
n: longint;
sum: longint;
mindif: longint;
f: text;
begin
assign(f,'C:\27-A.txt');
reset(f);
readln(f, n);
sum := 0;
mindif := 20001;
while not eof(f) do begin
readln(f, x, y);
if x > y then
sum := sum + x
else
sum := sum + y;
if (abs(x - y) < mindif) and (abs(x-y) mod 3 <> 0) then mindif := abs(x-y);
end;
if sum mod 3 <> 0 then
writeln(sum)
else
writeln(sum - mindif);
end.