Can anyone please explain it and give answer

Problem Statement
You have N light bulbs, however they are currently jumbled up in wires. When you start unravelling the bulbs, you observe that the situation can be described by a directed acyclic graph connecting the bulbs to each other.

Formally, you are given a directed graph consisting of N nodes (numbered 1 to N) and M edges such that it contains no cycles. A sequence of vertices (v1, v2, … vk) is called a path if there exists an edge from vi to vi+1 for every i such that 1 ≤ i ≤ k-1. A path (v1, v2, … vk) in the graph is called maximal, if for any vertex v, the sequence (v, v1, v2, … vk) or (v1, v2, … vk, v) do not represent paths in the graph.

Find the number of maximal paths in the graph.
Input
The first line contains two integers N and M.
The next M lines each contain two integers u and v – representing an edge from vertex u to vertex v.

Constraints:
1 ≤ N ≤ 105
0 ≤ M ≤ 2×105
1 ≤ u, v ≤ N
The graph does not contain any self-loops or multiple edges.
Output
Print a single integer – the number of maximal paths.
Example
Sample Input 1:
3 1
1 2

Sample Output 1:
2

Sample Explanation 1:
The two maximal paths are (1, 2) and (3).

Sample Input 2:
3 3
1 2
2 3
1 3

Sample Output 2:
2

Sample Explanation 1:
The two maximal paths are (1, 2, 3) and (1, 3).

Sample Input 3:
3 2
1 3
3 2

Sample Output 3:
1

Sample Explanation 3:
The only maximal path is (1, 3, 2).

It will be good if you provied the link of problem. (May be question from any codeing round.)