Feb. 21, 2014, 5:42 p.m. by Rosalind Team

**Topics**:
Graphs

Given: A positive integer

Return: For each graph, output a vertex from which all other vertices are reachable (if such a vertex exists) and "-1" otherwise.

Source: Algorithms by Dasgupta, Papadimitriou, Vazirani. McGraw-Hill. 2006.

2 3 2 3 2 2 1 3 2 3 2 1 2

3 -1