Hi, I need help with Excel - I don't get this program!

I'm searching for a solution to make Excel copy cells for me.

I have tried to make a sketch for what I'm looking for. Column A consists of numbers that doesn't change. Column B consists of numbers I can change. Column D is the output from column A & B.
Is there any way I can make this in Excel? If I type a number in column B it copies the number in column A from the same row into column D (next available row). I have no idea what to do and if it's possible when column D is so variating. I'm starting from scratch, so all ideas and answers will be helpful thank you!

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. | A | B | C | D |
1 |7 |2 | . |7 |
2 |8 |3 | . |7 |
3 | . | . | . |8 |
4 | . | . | . |8 |
5 | . | . | . |8 |
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