Problem:
Score : 100 points

Problem Statement

There is a directed graph G with N vertices and M edges.
The vertices are numbered 1, 2, \ldots, N, and for each i (1 \leq i \leq M), the i-th directed edge goes from Vertex x_i to y_i.
G does not contain directed cycles.

Find the length of the longest directed path in G.
Here, the length of a directed path is the number of edges in it.

Constraints

All values in input are integers.

2 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq x_i, y_i \leq N

All pairs (x_i, y_i) are distinct.

G does not contain directed cycles.

Input

Input is given from Standard Input in the following format:

N M
x_1 y_1
x_2 y_2
:
x_M y_M

Output

Print the length of the longest directed path in G.

Sample Input 1

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

Sample Output 1

3

The red directed path in the following figure is the longest:

Sample Input 2

6 3
2 3
4 5
5 6

Sample Output 2

2

The red directed path in the following figure is the longest:

Sample Input 3

5 8
5 3
2 3
2 4
5 2
5 1
1 4
4 3
1 3

Sample Output 3

3

The red directed path in the following figure is one of the longest:

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

/**
 * @author derrick20
 */
import java.io.*;
import java.util.*;

public class Main {
    public static void main(String args[]) throws Exception {
        FastScanner sc = new FastScanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int N = sc.nextInt();
        int M = sc.nextInt();
        adjList = new ArrayList[N];
        for (int i = 0; i < N; i++) {
            adjList[i] = new ArrayList<>();
        }
        for (int i = 0; i < M; i++) {
            int u = sc.nextInt() - 1;
            int v = sc.nextInt() - 1;
            adjList[u].add(v);
        }
        maxDist = new int[N];
        Arrays.fill(maxDist, -1);
        for (int i = 0; i < N; i++) {
            dfs(i);
        }
        out.println(ans);
        out.close();
    }

    static ArrayList<Integer>[] adjList;
    static int[] maxDist;
    static int ans;

    // Return the longest path we can get starting from this node
    static int dfs(int node) {
        if (maxDist[node] != -1) {
            // Memoizing!
            return maxDist[node];
        }
        else {
            int max = 0;
            for (int adj : adjList[node]) {
                max = Math.max(max, 1 + dfs(adj));
            }
            ans = Math.max(ans, max);
            return maxDist[node] = max;
        }
    }

    static class FastScanner {
        BufferedReader br;
        StringTokenizer st;

        FastScanner(InputStream s) {
            br = new BufferedReader(new InputStreamReader(s));
        }

        FastScanner(FileReader s) {
            br = new BufferedReader(s);
        }

        String next() throws IOException {
            while (st == null || !st.hasMoreTokens()) st = new StringTokenizer(br.readLine());
            return st.nextToken();
        }

        String nextLine() throws IOException { return br.readLine(); }

        double nextDouble() throws IOException { return Double.parseDouble(next()); }

        int nextInt() throws IOException { return Integer.parseInt(next()); }

        long nextLong() throws IOException { return Long.parseLong(next()); }
    }
}