#include <algorithm>
#include <climits>
#include <cmath>
#include <deque>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <regex>
#include <unordered_map>
#include <vector>

const int mod = 1e9 + 7;
const int kmax = 510000;
const int last_days[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

long long fact[kmax], fact_inv[kmax], inv[kmax];

void init_comb()
{
    fact[0] = fact[1] = 1;
    fact_inv[0] = fact_inv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < kmax; i++) {
        fact[i] = fact[i - 1] * i % mod;
        inv[i] = mod - inv[mod % i] * (mod / i) % mod;
        fact_inv[i] = fact_inv[i - 1] * inv[i] % mod;
    }
}

long long comb(int n, int r)
{
    if (n < r) {
        return 0;
    }

    if (n < 0 || r < 0) {
        return 0;
    }

    return fact[n] * (fact_inv[r] * fact_inv[n - r] % mod) % mod;
}

template <typename T, T N>
class UnionFind {
    T parent_[N];
    T rank_[N];
    T size_[N];

public:
    UnionFind();
    T Root(T idx);
    bool IsSame(T x, T y);
    void Unite(T x, T y);
    T GetSize(T idx);
};

template <typename T, T N>
UnionFind<T, N>::UnionFind()
{
    for (T i = 0; i < N; i++) {
        parent_[i] = i;
        rank_[i] = 0;
        size_[i] = 1;
    }
}

template <typename T, T N>
T UnionFind<T, N>::Root(T idx)
{
    return parent_[idx] == idx ? idx : parent_[idx] = Root(parent_[idx]);
}

template <typename T, T N>
bool UnionFind<T, N>::IsSame(T x, T y)
{
    return Root(x) == Root(y);
}

template <typename T, T N>
void UnionFind<T, N>::Unite(T x, T y)
{
    x = Root(x);
    y = Root(y);
    if (x == y) {
        return;
    }

    if (rank_[x] < rank_[y]) {
        parent_[x] = y;
        size_[y] += size_[x];
    } else {
        parent_[y] = x;
        size_[x] += size_[y];
        if (rank_[x] == rank_[y]) {
            rank_[x]++;
        }
    }
}

template <typename T, T N>
T UnionFind<T, N>::GetSize(T idx)
{
    return size_[Root(idx)];
}

long long pow_mod(long long n, long long p, long long m)
{
    if (p == 0) {
        return 1;
    }

    if (p % 2 == 0) {
        long long t = pow_mod(n, p / 2, m);
        return t * t % m;
    }
    return n * pow_mod(n, p - 1, m) % mod;
}

long long nCr_mod(long long n, long long r)
{
    long long x = 1;
    for (long long i = n - r + 1; i <= n; i++) {
        x *= i;
        x %= mod;
    }

    long long a = 1;
    for (long long i = 1; i <= r; i++) {
        a *= i;
        a %= mod;
    }

    long long y = pow_mod(a, mod - 2, mod) % mod;

    return x * y % mod;
}

bool is_prime(long long n)
{
    if (n == 1) {
        return false;
    }

    for (long long i = 2; i * i <= n; i++) {
        if (n % i == 0) {
            return false;
        }
    }

    return true;
}

bool is_leap(long long y)
{
    return (y % 4 == 0 && y % 100 != 0) || y % 400 == 0;
}

void next_day(long long& y, long long& m, long long& d)
{
    d++;

    if (d > last_days[m - 1] + (m == 2 && is_leap(y) ? 1 : 0)) {
        d = 1;
        m++;
    }

    if (m > 12) {
        y++;
        m = 1;
    }
}

long long fib(long long n)
{
    long long a = 0, b = 1;
    for (int i = 0; i < n; i++) {
        long long t = a;
        a = b;
        b = a + t;
    }

    return a;
}

// Note that the order of this function is O(n**n).
template <typename T>
std::vector<size_t> calculate_ranks(const std::vector<T>& v)
{
    std::vector<T> sorted = v;

    std::sort(sorted.begin(), sorted.end());

    std::map<T, long long> m;
    for (auto i = 0LU; i < v.size(); i++) {
        m.insert(std::make_pair(sorted[i], i));
    }

    std::vector<size_t> rank(v.size());

    for (auto i = 0LU; i < v.size(); i++) {
        rank[i] = m.find(v[i])->second + 1;
    }

    return rank;
}

std::map<long long, long long> prime_factors_and_num(long long n)
{
    std::map<long long, long long> m;
    for (int i = 2; i <= n; i++) {
        while (n % i == 0) {
            m[i]++;
            n /= i;
        }
    }

    return m;
}

inline long long calculate_sum(const std::vector<long long>& v)
{
    return std::accumulate(v.begin(), v.end(), 0LL);
}

int main()
{
    long long h, w;
    std::cin >> h >> w;

    std::vector<std::string> s(h);
    for (int i = 0; i < h; i++) {
        std::cin >> s[i];
    }

    auto f = [=](auto start_y, auto start_x, auto end_y, auto end_x) -> long long {
        std::queue<std::pair<std::pair<long long, long long>, long long>> q;
        q.push(std::make_pair(std::make_pair(start_y, start_x), 0));

        std::vector<std::vector<bool>> visited(h, std::vector<bool>(w, false));
        visited[start_y][start_x] = true;
        while (q.size()) {
            auto t = q.front();
            q.pop();

            auto y = t.first.first;
            auto x = t.first.second;
            auto count = t.second;

            if (y == end_y && x == end_x) {
                return count;
            }

            auto f = [=](auto y_, auto x_) -> bool {
                return y_ >= 0 && y_ < h && x_ >= 0 && x_ < w && s[y_][x_] != '#' && !visited[y_][x_];
            };

            if (f(y - 1, x)) {
                q.push(std::make_pair(std::make_pair(y - 1, x), count + 1));
                visited[y - 1][x] = true;
            }

            if (f(y + 1, x)) {
                q.push(std::make_pair(std::make_pair(y + 1, x), count + 1));
                visited[y + 1][x] = true;
            }

            if (f(y, x - 1)) {
                q.push(std::make_pair(std::make_pair(y, x - 1), count + 1));
                visited[y][x - 1] = true;
            }

            if (f(y, x + 1)) {
                q.push(std::make_pair(std::make_pair(y, x + 1), count + 1));
                visited[y][x + 1] = true;
            }
        }
    };

    long long max = 0;

    for (int sy = 0; sy < h; sy++) {
        for (int sx = 0; sx < w; sx++) {
            for (int dy = 0; dy < h; dy++) {
                for (int dx = 0; dx < w; dx++) {
                    if (sy == dy && sx == dx) {
                        continue;
                    }

                    if (s[sy][sx] == '#' || s[dy][dx] == '#') {
                        continue;
                    }

                    max = std::max(max, f(sy, sx, dy, dx));
                }
            }
        }
    }

    std::cout << max << std::endl;
}
