6749 - Next in line (C++)

https://www.acmicpc.net/problem/6749문제 : You know a family with three children. Their ages form an arithmetic sequence: the difference in ages between the middle child and youngest child is the same as the difference in ages between the oldest child and the middle child.

For example, their ages could be 5, 10 and 15, since both adjacent pairs have a difference of 5 years.Given the ages of the youngest and middle children, what is the age of the oldest child?문제가 영어라서 굉장히 당황했다.

영알못이기 때문에..굳이 번역하자면 첫째와 둘째 나이차이가 둘째랑 막내 나이차이와 같은데, 둘째랑 막내 나..........