Teacher:In this task, you're given a context, further information available on a particular linked term from the statement, and an answer term. Your job is to generate a question that can use the information provided to obtain the given answer. You should use the information on both context and link information to create the question. Note that the answer to the question should be exactly the given answer, and if the answer is none, the answer to the question shouldn't be obtainable from the context or linked information.
Teacher: Now, understand the problem? Solve this instance: Context: He previously served as a United Nations official for 32 years, most recently as the Resident Coordinator of the United Nations system in Ukraine, from 30 September 2004 until 31 March 2009, and previously in the same capacity in Serbia-Montenegro Link Information: According to the Peace of Riga, western Ukraine was incorporated into Poland, which in turn recognised the Ukrainian Soviet Socialist Republic in March 1919 Answer: Serbia
Student:
Which is the oldest country that O'Donnell served as the Resident Coordinator of the United Nations system for?