Problem L
The Wrath of Kahn
Topologically sorting the nodes of a directed graph (digraph) $G$ means putting the nodes in linear order
\[ v_0, v_1, v_2, v_3, \ldots \]such that whenever there is an edge from node $x$ to node $y$ in $G$, $x$ always precedes $y$ in the linear ordering. It is not hard to show that the nodes of a digraph can be topologically sorted if and only if the graph is acyclic (does not contain any directed cycles).
Kahn’s Algorithm is a wellknown topological sorting algorithm. Beginning with a digraph $G$ and an empty list $L$, it repeatedly executes the following three steps:

Let $S$ be the set of source nodes, i.e., the nodes with no incoming edges. If $S$ is empty, terminate the algorithm.

Let $\alpha $ be any node in $S$. Remove $\alpha $ and all its outgoing edges from $G$. If this removal of edges creates new source nodes, add these to $S$.

Insert $\alpha $ at the end of $L$.
Once Kahn’s Algorithm terminates, if $G$ is now empty, then $L$ contains the nodes of the initial graph in topologically sorted order (such an ordering may not be unique). Otherwise, $G$ has one or more cycles, and therefore topological sorting is impossible.
Regardless of the outcome of Kahn’s Algorithm, consider any iteration of Step $1$ above. If $S$ contains more than one node, then $\alpha $ can be any one of them, and the choice of $\alpha $ affects the composition of $S$ in later iterations, when further choices may be made that affect the composition of $S$ in yet later iterations, and so on. Taking into account all possible sequences of choices of $\alpha $ during the execution of Kahn’s Algorithm, what is the largest $S$ can ever be at the beginning of Step $1$?
Input
The input specifies a single directed graph. The first line of input contains two spaceseparated integers, $n$ and $m$ ($1 \leq n \leq 500$, $0 \leq m \leq n(n1)$), where $n$ is the number of nodes and $m$ is the number of edges. This is followed by $m$ lines, each containing two spaceseparated integers, $x$ and $y$ ($0 \leq x,y \leq (n1)$, $x \neq y$), indicating that there is a directed edge from node $x$ to node $y$, where nodes are indexed $0, 1, 2, \ldots , (n1)$. No edge will be listed more than once.
Output
Output a line containing a single integer: the largest possible size of $S$ at the beginning of any iteration of Step $1$ in the execution of Kahn’s Algorithm.
Sample Input 1  Sample Output 1 

4 3 0 1 1 2 2 3 
1 
Sample Input 2  Sample Output 2 

5 5 0 4 1 2 1 3 2 4 3 4 
3 