#include <algorithm>
#include <cassert>
#include <iostream>
#include <map>
#include <stack>
#include <tuple>
#include <unordered_set>
#include <vector>

using namespace std;

template <typename N = std::size_t, typename E = std::size_t>
class SparseGraph
{
public:
    using NodeType = N;
    using EdgeType = E;
    using NeighborListType = std::unordered_set<std::size_t>;

    SparseGraph(bool undirected = true) noexcept
        : undirected_(undirected)
    {
    }

    void push_node(NodeType n) noexcept
    {
        nodes_.push_back(n);
        neighbor_list_.emplace_back();
    }

    void connect(const std::size_t from, const std::size_t to, const EdgeType edge = EdgeType()) noexcept
    {
        if (undirected_)
        {
            neighbor_list_[from].insert(to);
            neighbor_list_[to].insert(from);
            edges_[std::make_pair(std::min(from, to), std::max(from, to))] = edge;
        }
        else
        {
            neighbor_list_[from].insert(to);
            edges_[std::make_pair(from, to)] = edge;
        }
    }

    void disconnect(const std::size_t from, const std::size_t to) noexcept
    {
        if (undirected_)
        {
            neighbor_list_[from].erase(to);
            neighbor_list_[to].erase(from);
            edges_.erase(std::make_pair(std::min(from, to), std::max(from, to)));
        }
        else
        {
            neighbor_list_[from].erase(to);
            edges_.erase(std::make_pair(from, to));
        }
    }

    const NodeType& node(const std::size_t index) const noexcept
    {
        return nodes_.at(index);
    }

    const EdgeType& edge(const std::size_t from, const std::size_t to) const noexcept
    {
        if (undirected_)
        {
            auto key = std::make_pair(std::min(from, to), std::max(from, to));
            return edges_.at(key);
        }
        else
        {
            auto key = std::make_pair(from, to);
            return edges_.at(key);
        }
    }

    const NeighborListType&
    neighbor(const std::size_t index) const noexcept
    {
        return neighbor_list_[index];
    }

    std::size_t size() const noexcept
    {
        return neighbor_list_.size();
    }

private:
    using EdgeMap = std::map<std::pair<std::size_t, std::size_t>, EdgeType>;

    std::vector<NeighborListType> neighbor_list_;

    EdgeMap edges_;

    std::vector<NodeType> nodes_;

    bool undirected_;
};

template <typename GraphType>
std::vector<std::size_t> topological_order(const GraphType& graph)
{
    std::vector<bool> decided(graph.size(), false);
    std::vector<std::size_t> order;

    for (int ns = 0; ns < graph.size(); ns++)
    {
        if (!decided[ns])
        {
            // (node, neihgbor index)
            stack<std::pair<int, int>> s;
            s.emplace(ns, 0);

            while (!s.empty())
            {
                int n, i;
                tie(n, i) = s.top();
                s.pop();

                if (!decided[n])
                {
                    // 帰りがけのタイミング
                    if (i == graph.neighbor(n).size())
                    {
                        decided[n] = true;
                        order.push_back(n);
                    }
                    else
                    {
                        s.emplace(n, i + 1);
                        for (auto next : graph.neighbor(n))
                        {
                            if (!decided[next])
                            {
                                s.emplace(next, 0);
                            }
                        }
                    }
                }
            }
        }
    }
    assert(order.size() == graph.size());
    return order;
}

int main()
{
    int n, m;
    cin >> n >> m;

    SparseGraph<> graph(false);
    for (int i = 0; i < n; i++)
    {
        graph.push_node(i);
    }

    for (int i = 0; i < m; i++)
    {
        int f, t;
        cin >> f >> t;
        graph.connect(f - 1, t - 1);
    }

    vector<int> dp(n, 0);
    for (auto node : topological_order(graph))
    {
        for (auto next : graph.neighbor(node))
        {
            dp[node] = max(dp[node], dp[next] + 1);
        }
    }

    cout << *max_element(dp.begin(), dp.end()) << std::endl;
}

Measured memory_bytes: 22188