In one formal texts I encountered this "в общей сложности" used when addressing the total amount of something. An example context below:
В общей сложности страны СНГ продемонстрировали падение числа изучающих русский ...
I was told that the в общей сложности is simply synonym for в итоге or в общем, even though there is the word сложность included. Does the word add any specific information to the context, and what is the logic behind this adverb? I assume this is used as a stable structure.
Additional question: are these kind of structures common in Russian? And are their meaning usually logically understandable, or are they mostly distinct cases that just need to be memorized?