Unique Source Reaching All Nodes

You are given an integer n and a directed graph with n nodes labeled 0 to n - 1, described by a 0-indexed list edges, where edges[i] = [u, v] is a directed edge from node u to node v.

Find the node that satisfies both of the following:

1. It has no incoming edges.
2. Every other node is reachable from it, either directly or through a chain of edges.

Return that node's index if exactly one node satisfies both conditions. Otherwise return -1.

Note that edges may contain duplicate edges, and a graph with a single node and no edges has a valid answer: node 0 vacuously reaches every other node.

Examples

Example 1

Input: n = 4, edges = [[0,1],[1,2],[2,3]]
Output: 0
Explanation: Node 0 has no incoming edges, and following the chain
0 -> 1 -> 2 -> 3 reaches every other node.

Example 2

Input: n = 4, edges = [[0,1],[2,1],[1,3]]
Output: -1
Explanation: Both node 0 and node 2 have no incoming edges, so there is no
single node satisfying both conditions.

Example 3

Input: n = 5, edges = [[2,0],[2,1],[2,3],[3,4]]
Output: 2
Explanation: Node 2 is the only node with no incoming edges. It reaches 0, 1,
and 3 directly, and reaches 4 through 3.