#include <algorithm>
#include <array>
#include <bitset>
#include <complex>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <valarray>
#include <vector>

#include <cassert>
#include <cctype>
#include <cstdio>
#include <cstdlib>
#include <cstring>

#include <chrono>
#include <random>
#include <thread>
#include <unordered_map>
#include <unordered_set>

using namespace std;

#define all(c) c.begin(), c.end()
#define repeat(i, n) for (int i = 0; i < static_cast<int>(n); i++)
#define debug(x) #x << "=" << (x)

#ifdef DEBUG
#define dump(x) std::cerr << debug(x) << " (L:" << __LINE__ << ")" << std::endl
#else
#define dump(x)
#endif

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v)
{
    os << "{";
    for (auto it = v.begin(); it != v.end(); ++it) {
        if (it != v.begin())
            os << ",";
        os << *it;
    }
    return os << "}";
}

template <typename A, typename B>
ostream& operator<<(ostream& os, const pair<A, B>& v)
{
    os << "{";
    os << v.first << ", " << v.second;
    return os << "}";
}

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

using point = complex<double>;
using ll = long long;

int solve(vector<vector<bool>>& wall, int sy, int sx)
{
    array<int, 4> dx = { -1,
        0,
        1,
        0 };
    array<int, 4> dy = { 0,
        -1,
        0,
        1 };

    int h = wall.size();
    int w = wall[0].size();

    const int INF = 10000000;
    vector<vector<int>> step(h, vector<int>(w, INF));

    step[sy][sx] = 0;

    queue<pair<int, int>> que;

    int curstep = 1;
    for (int i = 0; i < 4; i++) {
        int ny = sy + dy[i];
        int nx = sx + dx[i];

        if (0 <= ny && ny < h && 0 <= nx && nx < w && !wall[ny][nx]) {
            que.push({ ny, nx });
        }
    }

    while (!que.empty()) {
        queue<pair<int, int>> nex;

        while (!que.empty()) {
            auto p = que.front();
            que.pop();

            int y = p.first;
            int x = p.second;
            if (step[y][x] != INF)
                continue;
            step[y][x] = curstep;

            for (int i = 0; i < 4; i++) {
                int ny = y + dy[i];
                int nx = x + dx[i];
                if (0 <= ny && ny < h && 0 <= nx && nx < w && !wall[ny][nx]) {
                    nex.push({ ny, nx });
                }
            }
        }

        curstep++;
        que = nex;
    }

    int ret = 0;
    for (int y = 0; y < h; y++) {
        for (int x = 0; x < w; x++) {
            if (!wall[y][x])
                ret = max(ret, step[y][x]);
        }
    }
    return ret;
}

int my_main(int argc, char** argv)
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    int h, w;
    cin >> h >> w;

    vector<vector<bool>> wall(h, vector<bool>(w));
    for (int i = 0; i < h; i++) {
        string s;
        cin >> s;
        for (int j = 0; j < w; j++) {
            wall[i][j] = s[j] == '#';
        }
    }

    int ret = 0;
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            if (!wall[i][j])
                ret = max(ret, solve(wall, i, j));
        }
    }
    cout << ret << endl;
    return 0;
}

// ----------------------------------------------------------------------------
// Test driver

#ifdef DEBUG
#define MAY_RUN_TESTCASE
#endif

#ifdef MAY_RUN_TESTCASE
#include <fstream>

#if __has_include(<unistd.h>)
// Mac, Linux
#include <unistd.h>
#elif __has_include(<io.h>)
// Windows
#include <io.h>
#else
// Other?
#error "unistd.h or io.h not found"
#endif

bool test_file_exists(const std::string& str)
{
    std::ifstream fs(str);
    return fs.is_open();
}

#endif

int main(int argc, char** argv)
{
#ifndef MAY_RUN_TESTCASE
    return my_main(argc, argv);
#else
    if (argc == 1) {
        return my_main(argc, argv);
    } else if (argc == 2) {

        char* stdin_file = argv[1];
        freopen(stdin_file, "r", stdin);
        return my_main(argc, argv);

    } else if (argc == 5) {

        std::string stdin_file = argv[1];
        std::string expected_file = argv[2];
        std::string stdout_file = argv[3];
        std::string stderr_file = argv[4];

        if (!test_file_exists(stdin_file)) {
            std::cerr << stdin_file << " not found" << std::endl;
            return 3;
        }

        if (!test_file_exists(expected_file)) {
            std::cerr << expected_file << " not found" << std::endl;
            return 3;
        }

        int original_stdin = dup(fileno(stdin));
        int original_stdout = dup(fileno(stdout));
        int original_stderr = dup(fileno(stderr));

        freopen(stdin_file.c_str(), "r", stdin);
        freopen(stdout_file.c_str(), "w", stdout);
        freopen(stderr_file.c_str(), "w", stderr);

        int ret = my_main(argc, argv);

        fflush(stdout);
        fflush(stderr);

        dup2(original_stderr, fileno(stderr));
        dup2(original_stdout, fileno(stdout));
        dup2(original_stdin, fileno(stdin));

        if (ret != 0) {
            std::cerr << "main returns " << ret << std::endl;
            return ret;
        }

        std::ifstream inp(stdin_file);
        std::ifstream out(stdout_file);
        std::ifstream err(stderr_file);
        std::ifstream exp(expected_file);

        // Clear is needed if the file is empty.
        std::cout << "----- input -----" << std::endl;
        std::cout << inp.rdbuf() << std::endl;
        std::cout.clear();
        std::cout << "-----------------" << std::endl
                  << std::endl;

        std::cout << "----- output ----" << std::endl;
        std::cout << out.rdbuf() << std::endl;
        std::cout.clear();
        std::cout << "-----------------" << std::endl
                  << std::endl;

        std::cout << "---- expected ---" << std::endl;
        std::cout << exp.rdbuf() << std::endl;
        std::cout.clear();
        std::cout << "-----------------" << std::endl
                  << std::endl;

        std::cout << "----- stderr ----" << std::endl;
        std::cout << err.rdbuf() << std::endl;
        std::cout.clear();
        std::cout << "-----------------" << std::endl;

        inp.seekg(0);
        out.seekg(0);
        exp.seekg(0);
        err.seekg(0);

        std::string output_str, expected_str;
        {
            std::stringstream output_ss;
            output_ss << out.rdbuf();
            output_str = output_ss.str();

            std::stringstream expected_ss;
            expected_ss << exp.rdbuf();
            expected_str = expected_ss.str();
        }

        // Remove trailing spaces
        output_str.erase(output_str.find_last_not_of(" \n\r\t") + 1);
        expected_str.erase(expected_str.find_last_not_of(" \n\r\t") + 1);

        if (output_str == expected_str)
            return 0;
        else
            return 1;
    }
    return 1;
#endif
}


Measured memory_bytes: 256