Could someone tell me how to do the below without opening the file? (eg in sed or awk)

I have a file with the contenst below:

$ more file1.txt
10
AAA;
200
BBB;
3
CCC;


I want to delete the carriage return of one line above the line which has ";" at the end to get the below:

10 AAA;
200 BBB;
3 CCC;

Any help will be appreciated.