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 <algorithm>
#include <boost/optional.hpp>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#define REP(i, n) for(int i = 0, i##_MACRO = (n); i < i##_MACRO; i++)
#define RANGE(i, a, b) for(int i = (a), i##_MACRO = (b); i < i##_MACRO; i++)
#define EACH(e, a) for(auto&& e : a)
#define ALL(a) std::begin(a), std::end(a)
#define RALL(a) std::rbegin(a), std::rend(a)
#define FILL(a, n) memset((a), n, sizeof(a))
#define FILLZ(a) FILL(a, 0)
#define INT(x) (static_cast<int>(x))

using namespace std;

using ll = long long;
using VI = vector<int>;
using VI2D = vector<vector<int>>;

constexpr int INF = 2e9;
constexpr double EPS = 1e-10;
constexpr double PI = acos(-1.0);

constexpr int dx[] = {-1, 0, 1, 0};
constexpr int dy[] = {0, -1, 0, 1};

template <typename T>
constexpr int sign(T x) {
	return x < 0 ? -1 : x > 0 ? 1 : 0;
}

template <>
constexpr int sign(double x) {
	return x < -EPS ? -1 : x > EPS ? 1 : 0;
}

template <typename T, typename U>
constexpr void chmax(T& m, U x) {
	m = max(m, x);
}

template <typename T, typename U>
constexpr void chmin(T& m, U x) {
	m = min(m, x);
}

template <typename T>
constexpr T square(T x) {
	return x * x;
}

std::vector<int> topological_sort(std::vector<std::vector<int>> edges,
								  std::vector<int> indegrees) {
	int n = edges.size();
	std::vector<int> sorted;
	std::vector<bool> is_used(n, false);
	std::queue<int> q;
	for(int i = 0; i < n; i++) {
		if(indegrees[i] == 0) {
			q.push(i);
		}
	}

	while(!q.empty()) {
		int node = q.front();
		q.pop();
		sorted.push_back(node);
		is_used[node] = true;
		for(int& e : edges[node]) {
			indegrees[e]--;
			if(!is_used[e] && indegrees[e] == 0) {
				q.push(e);
			}
		}
	}
	return sorted;
}

int main() {
	int n, m;
	cin >> n >> m;
	VI2D edges(n);
	VI indegrees(n, 0);
	REP(i, m) {
		int x, y;
		cin >> x >> y;
		edges[x - 1].push_back(y - 1);
		indegrees[y - 1]++;
	}
	int result = 0;
	VI sorted = topological_sort(edges, indegrees);
	VI dp(n, 0);
	REP(i, n) {
		EACH(e, edges[sorted[i]]) {
			dp[e] = max(dp[e], dp[sorted[i]] + 1);
			chmax(result, dp[e]);
		}
	}
	cout << result << endl;
	return 0;
}