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 C++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
vector<vector<int>> G;
vector<int> path_length;
int rec(int cv) {
    if (path_length[cv] != -1) return path_length[cv];
    int ret = 0;
    for (auto nv: G[cv]) {
        ret = max(ret, rec(nv) + 1);
    }
    return path_length[cv] = ret;
}

int main() {
    int n, m;
    cin >> n >> m;
    G.assign(n, vector<int>());
    for (int i = 0; i < m; i++) {
        int x, y;
        cin >> x >> y;
        x--, y--;
        G[x].push_back(y);
    }
    path_length.assign(n, -1);
    int ans = 0;
    for (int i = 0; i < n; i++) ans = max(ans, rec(i));
    cout << ans << endl;
    return 0;
}