I booked my flight today!
I had been waiting for one more document from the States, but it turned out that I don't need that for entry into the US...
I thought June is not a high season for sight-seeing or anything, but when I asked the availability to several travel agencies, they said the flights on this weekend are all full....
I was so worried if I need to wait another week, but I was lucky. I found an available flight on the internet this morning and booked it!
I'll be leaving on 16.