#include <algorithm>
#include <iostream>
#include <fstream>
#include <sstream>
#include <bitset>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <list>
#include <map>
#include <set>
#include <cmath>
#include <limits>
#include <numeric>
#include <iomanip>
#include <cstdlib>
#include <iterator>
#include <functional>
#include <unordered_map>
#include <unordered_set>

using namespace std;

typedef unsigned                    ui;
typedef int                         si;
typedef long                        sl;
typedef long long                   ll;
typedef unsigned long long          ull;

// ----------------

template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
  return '"' + s + '"';
}

string to_string(const char& ch) {
    string result;
    result.append("'");
    result.push_back(ch);
    result.append("'");
    return result;
}

string to_string(const char* s) {
  return to_string((string) s);
}

string to_string(bool b) {
  return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

// ----------------

template <typename T>
struct PairHash {
  size_t operator() (const pair<T, T>& v) const {
    return hash<ll>()((static_cast<ll>(v.first)) ^ (static_cast<ll>(v.second)) << 32);
  }
};

class Item {
public:
    ll I = -1;
    ll J = -1;
    ll D = -1;
};

int main(int argc, char *argv[])
{
    std::ios_base::sync_with_stdio(false);

    ll n, m;
    cin >> n >> m;

    vector<string> grid(n);
    for (int i = 0; i < n; ++i) {
        cin >> grid[i];
    }

    pair<int, int> start = { -1, -1 };
    for (int i = 0; i < n; ++i) {
        if (start.first >= 0) {
            break;
        }

        for (int j = 0; j < m; ++j) {
            if (grid[i][j] != '#') {
                start = pair<int, int>(i, j);
                break;
            }
        }
    }

    if (start.first < 0) {
        cout << 0 << endl;
        return 0;
    }

    vector<vector<bool>> used(n, vector<bool>(m, false));

    const auto find_fartest = [&](const pair<int, int>& start) -> Item {
        queue<Item> q;

        Item s{start.first, start.second, 0};
        q.push(s);

        debug(start);

        Item answer = s;
        while (!q.empty()) {
            Item cur = q.front();
            q.pop();

            if (grid[cur.I][cur.J] == '#') {
                continue;
            }

            if (used[cur.I][cur.J]) {
                continue;
            }

            used[cur.I][cur.J] = true;
            if (cur.D > answer.D) {
                answer = cur;
            }

            if (cur.I > 0 && grid[cur.I - 1][cur.J] != '#') {
                q.emplace(Item{cur.I - 1, cur.J, cur.D + 1});
            }
            if (cur.I + 1 < n && grid[cur.I + 1][cur.J] != '#') {
                q.emplace(Item{cur.I + 1, cur.J, cur.D + 1});
            }
            if (cur.J > 0 && grid[cur.I][cur.J - 1] != '#') {
                q.emplace(Item{cur.I, cur.J - 1, cur.D + 1});
            }
            if (cur.J + 1 < m && grid[cur.I][cur.J + 1] != '#') {
                q.emplace(Item{cur.I, cur.J + 1, cur.D + 1});
            }
        }

        return answer;
    };

    ll ans = 0;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {
            if (grid[i][j] != '#') {
                used.assign(n, vector<bool>(m, false));
                Item found = find_fartest(pair<int, int>(i, j));

                ans = max(ans, found.D);
            }
        }
    }

    cout << ans << endl;

    return 0;
}


Measured memory_bytes: 256