Find Middle Course

You are given a list of pairs, where each pair represents a course dependency in the format [prerequisite, course]. This means the prerequisite must be taken before the course. The input is guaranteed to form a single, unbranched, continuous chain of courses. The total number of courses in this chain is always odd, so there is exactly one middle course. Every course appears at most once as a prerequisite (except the very first course in the chain) and at most once as a subsequent course (except the very last course).

Your task is to find and return the middle course of this chain.

Constraints:

• The input always forms a single, continuous chain with no branches or merges.
• The total number of courses is odd.
• Each course appears uniquely in its role as a prerequisite or as a subsequent course, except for the start and end courses.

Example 1:

Example 2:

Example 3:

Examples

Example 1

Input: [["Chemistry","Biology"],["Biology","ComputerScience"],["Math","Physics"],["Physics","Chemistry"]]
Output: "Chemistry"

Example 2

Input: [["Intro","Data"],["Data","Algo"],["Algo","Systems"],["Systems","AI"]]
Output: "Algo"