program p1;
label 1;
var i,n,sum:integer;
    prod:char;
 
begin
1:write('N= '); readln(n);
sum:=0;
i:=1;
while (i<n+1) do begin
sum:=sum+i;
i:=i+3;
end;
 
writeln('Summa= ',sum);
 
write('Vi xotite prodoljiti? (y/n) ');  readln(prod);
if prod='y' then goto 1;
readln;
end.