#include <bits/stdc++.h>
#define range(i, l, r) for(long long int (i) = (l); (i) < (r); (i)++)
#define reversed_range(i, l, r) for (long long int (i) = (r) - 1; (i) >= l; (i)--)
using namespace std;
template <typename T>
using vec = vector<T>;
using lint = long long;
using ulint = unsigned long long;
using pint = pair<int, int>;
using plint = pair<lint, lint>;

template <typename S, typename T>
ostream& operator <<(ostream& os, pair<S, T> p) {
    os << "(";
    os << p.first << ", " << p.second;
    return os << ")";
}

template <typename T>
ostream& operator <<(ostream& os, vec<T> v) {
    os << "[";
    if (v.size() == 0) return os << "]";
    for (int i = 0; i < v.size() - 1; i++) {
        os << v.at(i) << ", ";
    }
    return os << v.at(v.size() - 1) << "]";
}

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

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

template <typename K, typename V>
ostream& operator <<(ostream& os, map<K, V> m) {
    os << "{";
    if (m.begin() == m.end()) return os << "}";
    auto it_first_item = m.begin();
    os << it_first_item->first << ": " << it_first_item->second;
    for (auto it = ++it_first_item; it != m.end(); it++) {
        os << ", " << it->first << ": " << it->second;
    }
    return os << "}";
}

template <typename K, typename V>
ostream& operator <<(ostream& os, unordered_map<K, V> m) {
    os << "{";
    if (m.begin() == m.end()) return os << "}";
    auto it_first_item = m.begin();
    os << it_first_item->first << ": " << it_first_item->second;
    for (auto it = ++it_first_item; it != m.end(); it++) {
        os << ", " << it->first << ": " << it->second;
    }
    return os << "}";
}

lint pow(lint num, lint e, lint MOD) {
    lint res = 1;
    lint cur_num = num;
    while (e) {
        if (e & 1) {
            res *= cur_num;
            res %= MOD;
        }
        cur_num *= cur_num;
        cur_num %= MOD;
        e >>= 1;
    }
    return res;
}

int main() {
    cin.tie(0); cout.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    lint H, W; cin >> H >> W;
    vec<string> maze(H);
    range(i, 0, H) cin >> maze.at(i);
    lint ans = 0;
    vec<lint> dy = {-1, 1, 0, 0}, dx = {0, 0, -1, 1};
    auto max_dist = [&](lint sy, lint sx) {
        vec<vec<lint>> d(H, vec<lint>(W, -1));
        d.at(sy).at(sx) = 0;
        deque<plint> que;
        que.emplace_back(sy, sx);
        while (!que.empty()) {
            lint y = que.front().first, x = que.front().second;
            que.pop_front();
            range(k, 0, 4) {
                lint ny = y + dy.at(k), nx = x + dx.at(k);
                if (ny < 0 or ny >= H or nx < 0 or nx >= W) continue;
                else if (d.at(ny).at(nx) != -1) continue;
                else if (maze.at(ny).at(nx) == '#') continue;
                else {
                    d.at(ny).at(nx) = d.at(y).at(x) + 1;
                    que.emplace_back(ny, nx);
                }
            }
        }
        lint res = 0;
        range(i, 0, H) range(j, 0, W) res = max(res, d.at(i).at(j));
        return res;
    };
    range(i, 0, H) range(j, 0, W) {
        if (maze.at(i).at(j) == '#') continue;
        ans = max(ans, max_dist(i, j));
    }
    cout << ans << "\n";
}

Measured memory_bytes: 256