Long Term vs. Short Term Capital Gains

Hey all,

I'm new to investing, and I'm having a hard time wrapping my small brain around some of the tax aspects of earnings. And my question is so specific, I can't seem to find any answers through research.

I understand that long term capital gains are taxed at 15% and short term capital gains are taxed at normal income tax rates.

Let's say I have 500 shares in a company stock. Ideally, I want to hold these for 1 year +1 day to avoid paying income tax. But I decide to sell 100 of the shares early and pay the income tax. Then a few days later I buy the shares back (hopefully at a profit).

So I'm back to 500 shares.

Fast forward a year, I've owned 400 shares for a year and a day, and 100 shares for less than a year. So my question: If I sell 400 of my shares in the company, will they all be considered LTCGs or will I also be paying a portion in STCGs.

Thanks for the help, and I'm sorry if there's something extremely obvious I'm not seeing.