`scanf`는 리턴값으로 성공적으로 입력받은 인자의 개수를 리턴한다.

https://cplusplus.com/reference/cstdio/scanf/

On success, the function returns the number of items of the argument list successfully filled. This count can match the expected number of items or be less (even zero) due to a matching failure, a reading error, or the reach of the end-of-file.