#define _CRT_SECURE_NO_WARNINGS

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <cmath>
#include <algorithm>
#include <iomanip>
#include <utility>
#include <ctime>
#include <map>
#include <set>
#include <unordered_set>

using namespace std;

#define endl "\n"
#define age first
#define pos second
#define Z third
#define mp make_pair
#define mt make_triple

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;

const ld PI = 3.141592653589793;
const ld E = 2.71828182845904523;
const ll LLINF = 4000000009000000099;
const ll INF = 1009000099;
const ll MOD7 = 1000000007;
const ll MOD9 = 1000000009;

template<class A, class B, class C> struct triple { A first; B second; C third; triple(A, B, C); triple(); };
template<class A, class B, class C> triple<A, B, C>::triple(A a, B b, C c) { first = a; second = b; third = c; }
template<class A, class B, class C> triple<A, B, C>::triple() { }
template<class A, class B, class C> triple<A, B, C> make_triple(A a, B b, C c) { triple<A, B, C> t(a, b, c); return t; }
template<class A, class B, class C> bool operator<(triple<A, B, C> a, triple<A, B, C> b) { if (a.first != b.first) return a.first < b.first; if (a.second != b.second) return a.second < b.second; if (a.third != b.third) return a.third < b.third; }
template<class T> T inline sqr(T a) { return a * a; }

ll bpow(ll a, ll d, ll mod) { if (d == 0) return 1; ll t = bpow(a, d / 2, mod); t = (t * t) % mod; if (d & 1) t = (t * a) % mod; return t; }
ll bpow(ll a, ll d) { if (d == 0) return 1; ll t = bpow(a, d / 2); t = (t * t); if (d & 1) t = (t * a); return t; }
ll gcd(ll a, ll b) { while (a && b) if (a > b) a %= b; else b %= a; return a + b; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
ll C(ll n, ll k) { ll res = 1; for (int i = 1; i <= k; ++i) res = res * (n - k + i) / i; return res; }
ll C(ll n, ll k, ll mod) { ll u = 1, d = 1; for (int i = 2; i <= n; ++i) u = (u * i) % mod; int x = max(k, n - k), z = min(k, n - k); for (int i = 2; i <= x; ++i) { d = (d * i) % mod; if (i <= z) d = (d * i) % mod; } d = bpow(d, mod - 2, mod); ll res = (u * d) % mod; return res; }

using namespace std;

void solve1()
{
	int n; cin >> n;
	vector<pair<int, int>> v(n);
	for (int i = 0; i < n; ++i)
	{
		cin >> v[i].first;
		v[i].second = i;
	}
	sort(v.begin(), v.end());

	vector<set<int>> cycles;
	set<int> used;
	for (int i = 0; i < n; ++i)
	{
		set<int> s;
		int idx = i;
		while (used.find(v[idx].first) == used.end())
		{
			used.insert(v[idx].first);
			s.insert(v[idx].first);
			idx = v[idx].second;
		}

		if (s.size() > 1)
			cycles.push_back(s);
	}
	int res = 0, glMin = v.front().first;
	for (auto cycle : cycles)
	{
		int sum = 0, locMin = *cycle.begin();
		for (auto it = cycle.begin(); it != cycle.end(); ++it)
			sum += *it;
		res += min(sum + locMin * (cycle.size() - 2),
			sum + locMin + glMin * (cycle.size() + 1));
	}

	cout << res << endl;
}

int shortest(const vector<string>& field, pair<int, int> s, pair<int, int> f)
{
	int h = field.size(),
		w = field.front().size();
	vector<vector<int>> d(field.size(), vector<int>(field.front().size(), INF));
	d[s.first][s.second] = 0;
	queue<pair<int, int>> q;
	q.push(s);
	while (d[f.first][f.second] == INF)
	{
		auto top = q.front(); q.pop();

		for (int dx = -1; dx <= 1; ++dx)
			for (int dy = -1; dy <= 1; ++dy)
				if (abs(dx) + abs(dy) == 1)
				{
					int nx = top.first + dx,
						ny = top.second + dy;
					if (nx >= 0 && nx < h &&
						ny >= 0 && ny < w && d[nx][ny] == INF &&
						field[nx][ny] == '.')
					{
						d[nx][ny] = d[top.first][top.second] + 1;
						q.push({nx, ny});
					}
				}
	}
	return d[f.first][f.second];
}

void solve()
{
	int h, w; cin >> h >> w;
	vector<string> field(h);
	for (int i = 0; i < h; ++i)
		cin >> field[i];
	int ans = 0;
	for (int i = 0; i < h; ++i)
		for (int j = 0; j < w; ++j)
			for (int i1 = 0; i1 < h; ++i1)
				for (int j1 = 0; j1 < w; ++j1)
					if (field[i][j] == field[i1][j1]
						&& field[i][j] == '.')
						ans = max(ans, shortest(field, { i, j }, { i1, j1 }));
	cout << ans;
}

int main() {
	cin.tie(0);
	cout.tie(0);
	ios::sync_with_stdio(0);
#ifdef _DEBUG
	freopen("in.txt", "r", stdin);
	//freopen("out.txt", "w", stdout);
#endif
	solve();
	return 0;
}

Measured memory_bytes: 256