if 0 == y%4 then
    if 0 == y%100 then
        if 0 == y%400 then
            io.write(y, " is a leap year");
        else
            io.write(y, " is not a leap year");
        end
    else
        io.write(y, " is a leap year");
    end
else
    io.write(y, " is not a leap year");
end