/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author aryssoncf
 */



// Taken from: https://github.com/bqi343/USACO/blob/master/Implementations/content/contest/templateShort.cpp

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pi;
typedef vector<int> vi;
typedef vector<ull> vul;
typedef vector<ll> vl;
typedef vector<pi> vpi;

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define F0R(i, a) FOR(i,0,a)
#define ROF(i, a, b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i, a) ROF(i,0,a)
#define trav(a, x) for (auto& a: x)

#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rsz resize

#define mp make_pair
#define pb push_back
#define f first
#define s second

const int MOD = 1e9 + 7; // 998244353; // = (119<<23)+1
const int MX = 2e5 + 5;

template<class T>
bool ckmin(T &a, const T &b) {
    return a > b ? a = b, 1 : 0;
}

template<class T>
bool ckmax(T &a, const T &b) {
    return a < b ? a = b, 1 : 0;
}

mt19937 rng((uint32_t) chrono::steady_clock::now().time_since_epoch().count());


//
// Created by aryssoncf on 28/10/2019.
//

#include <type_traits>


#include <iterator>
#include <utility>

template<typename T>
constexpr auto hasBegin(int) -> decltype(std::begin(std::declval<T>()), true) {
    return true;
}

constexpr bool hasBegin(...) {
    return false;
}

template<typename T>
using IsContainer = std::integral_constant<bool, hasBegin<T>(0)>;


#ifndef JHELPER_EXAMPLE_PROJECT_IO_HPP
#define JHELPER_EXAMPLE_PROJECT_IO_HPP

#endif //JHELPER_EXAMPLE_PROJECT_IO_HPP

template<typename T>
std::istream &operator>>(std::istream &is, std::vector<T> &vec) {
    for (T &element: vec) {
        is >> element;
    }
    return is;
}

template<typename T, typename U>
std::istream &operator>>(std::istream &is, std::pair<T, U> &p) {
    is >> p.first;
    is >> p.second;
    return is;
}

template<class X, class Y>
std::ostream &operator<<(std::ostream &os, std::pair<X, Y> const &p) {
    return os << p.first << " " << p.second;
}

template<class Ch, class Tr, class Container>
std::basic_ostream<Ch, Tr> &operator<<(std::basic_ostream<Ch, Tr> &os, Container const &x) {
    bool first = true;
    for (auto &y : x) {
        if (first) {
            first = false;
        } else {
            os << " ";
        }
        os << y;
    }
    return os;
}


#include <vector>
#include <numeric>
#include <algorithm>

using namespace std;

template<typename T>
class VectorVectorBaseGraph {
public:
    using Edge = T;
    using Vertex = std::size_t;

    explicit VectorVectorBaseGraph(std::size_t n = 0) : g(n) {
    }

    template<typename U>
    using Collection = std::vector<U>;

    template<typename U>
    Collection<U> getCollection(const U &defaultValue) const {
        return Collection<U>(g.size(), defaultValue);
    }

    std::vector<std::size_t> vertices() const {
        std::vector<std::size_t> order(g.size());
        std::iota(order.begin(), order.end(), 0);
        return order;
    }

    const std::vector<Edge> &outgoingEdges(Vertex vertex) const {
        return g[vertex];
    }

    vector<Vertex> toposort() {
        int n = g.size();
        vector<bool> used(n);
        vector<Vertex> order;
        for (int i = 0; i < n; i++)
            if (!used[i])
                dfs(used, order, i);
        reverse(order.begin(), order.end());
        return order;
    }

    virtual Vertex endOfEdge(Vertex, Edge) const = 0;

protected:
    std::vector<std::vector<Edge>> g;

private:
    void dfs(vector<bool> &used, vector<Vertex> &order, int u) {
        used[u] = true;
        for (Edge ed : outgoingEdges(u)) {
            Vertex v = endOfEdge(u, ed);
            if (!used[v])
                dfs(used, order, v);
        }
        order.push_back(u);
    }
};


class SimpleGraph : public VectorVectorBaseGraph<std::size_t> {
public:
    explicit SimpleGraph(std::size_t n = 0) : VectorVectorBaseGraph(n) {}

    Vertex endOfEdge(Vertex, Edge edge) const {
        return edge;
    }

    void addEdge(Vertex from, Vertex to) {
        g[from].push_back(to);
    }
};


#include <queue>

template<typename Graph>
std::vector<int> bfsDistances(const Graph &graph, const typename Graph::Vertex &from) {
    auto result = graph.getCollection(-1);
    std::queue<typename Graph::Vertex> q;
    q.push(from);
    result[from] = 0;
    while (!q.empty()) {
        const auto &v = q.front();
        for (const auto &e: graph.outgoingEdges(v)) {
            const auto &to = graph.endOfEdge(v, e);
            if (result[to] == -1) {
                result[to] = result[v] + 1;
                q.push(to);
            }
        }
        q.pop();
    }
    return result;
}


using namespace std;

/*
PROG:
LANG: C++
*/

class DMazeMaster {
    const vi DX = {-1, 0, 0, 1};
    const vi DY = {0, -1, 1, 0};

public:
    void solve(std::istream &in, std::ostream &out) {
        int n, m;
        in >> n >> m;
        vector<string> grid(n);
        in >> grid;
        SimpleGraph graph(n * m);
        FOR(i, 0, n) {
            FOR(j, 0, m) {
                if (grid[i][j] == '.') {
                    FOR(k, 0, 4) {
                        int i2 = i + DX[k], j2 = j + DY[k];
                        if (i2 >= 0 and i2 < n and j2 >= 0 and j2 < m and grid[i2][j2] == '.') {
                            graph.addEdge(i * m + j, i2 * m + j2);
                        }
                    }
                }
            }
        }
        int res = 0;
        FOR(i, 0, n * m) {
            vi dist = bfsDistances(graph, i);
            ckmax(res, *max_element(all(dist)));
        }
        out << res << "\n";
    }

    void setup() {}
};


int main() {
    std::ios_base::sync_with_stdio(false);
    DMazeMaster solver;
    solver.setup();
    std::istream &in(std::cin);
    std::ostream &out(std::cout);
    in.tie(nullptr);
    out << std::fixed;
    out.precision(20);
    solver.solve(in, out);
    return 0;
}


Measured memory_bytes: 256