//author: zekigurbuz (Zeki Gurbuz) 2020
#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
typedef double db;
typedef string str;

typedef pair<int,int> pi;
typedef pair<ll,ll> pl;
typedef pair<db,db> pd;
typedef pair<ld,ld> pld;
#define mp make_pair
#define fi first
#define se second

typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<db> vd;
typedef vector<ld> vld;
typedef vector<str> vs;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<pd> vpd;
typedef vector<pld> vpld;

typedef queue<bool> qb;
typedef queue<int> qi;
typedef queue<ll> ql;
typedef queue<db> qd;
typedef queue<ld> qld;
typedef queue<str> qs;
typedef queue<pi> qpi;
typedef queue<pl> qpl;
typedef queue<pd> qpd;
typedef queue<pld> qpld;

typedef map<str,int> msi;
typedef map<int,int> mii;
typedef map<int,vi> mivi;
typedef map<int,vpi> mivpi;

#define pq priority_queue
typedef pq<int> pqi;
typedef pq<ll> pql;
typedef pq<db> pqd;
typedef pq<ld> pqld;
typedef pq<str> pqs;

typedef set<int> si;
typedef set<ll> sl;
typedef set<db> sdb;
typedef set<ld> sld;
typedef set<str> ss;
typedef set<pi> spi;
typedef set<pl> spl;
typedef set<pd> spd;
typedef set<pld> spld;

#define us unordered_set
typedef us<int> usi;
typedef us<ll> usl;
typedef us<db> usdb;
typedef us<ld> usld;
typedef us<str> usstr;
typedef us<pi> uspi;
typedef us<pl> uspl;
typedef us<pd> uspd;
typedef us<pld> uspld;

#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rall(x) (x).rbegin(), (x).rend()
#define rsz resize
#define ins insert
#define ft front()
#define bk back()
#define pf push_front
#define pb push_back
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound

#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b) - 1; i >= (a); i--
#define R0F(i,a) ROF(i,0,a)
#define trav(a,x) for (auto& a : x)

const int MOD = 1e9+7; //998244353
const int MX = 2e5+5;
const ll INF = 1e18;
const ld PI = 4*atan((ld)1);
const int dx[4] = {0,1,0,-1}, dy[4] = {1,0,-1,0};

template<class T> bool ckmin(T& a, const T& b) {
	return a > b ? a = b, 1 : 0;
}

template<class T> bool ckmax(T& a, const T& b) {
	return a < b ? a = b, 1 : 0;
}

mt19937 rng((uint32_t)chrono::steady_clock::now().time_since_epoch().count());

#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

namespace input {
	template<class T> void re(complex<T>& x);
	template<class T1, class T2> void re(pair<T1,T2>& p);
	template<class T> void re(vector<T>& a);
	template<class T, size_t SZ> void re(array<T,SZ>& a);
	
	template<class T> void re(T& x) { cin >> x; }
	void re(double& x) { string t; re(t); x = stold(t); }
	void re(ld& x) { string t; re(t); x = stold(t); }
	template<class T, class... Ts> void re(T& t, Ts&... ts) {
		re(t); re(ts...);
	}

	template<class T> void re(complex<T>& x) { T a,b; re(a,b); x = cd(x,b); }
	template<class T1, class T2> void re(pair<T1,T2>& p) { re(p.fi, p.se); }
	template<class T> void re(vector<T>& a) { F0R(i,sz(a)) re(a[i]); }
	template<class T, size_t SZ> void re(array<T,SZ>& a) { F0R(i,SZ) re(a[i]); }
}

using namespace input;

namespace output {
	void pr(int x) { cout << x; }
	void pr(long x) { cout << x; }
	void pr(ll x) { cout << x; }
	void pr(unsigned x) { cout << x; }
	void pr(unsigned long x) { cout << x; }
	void pr(unsigned long long x) { cout << x; }
	void pr(float x) { cout << x; }
	void pr(double x) { cout << x; }
	void pr(ld x) { cout << x; }
	void pr(char x) { cout << x; }
	void pr(const char* x) { cout << x; }
	void pr(const string& x) { cout << x; }
	void pr(bool x) { pr(x ? "true" : "false"); }
	template<class T> void pr(const complex<T>& x) { cout << x; }
	
	template<class T1, class T2> void pr(const pair<T1,T2>& x);
	template<class T> void pr(const T& x);

	template <class T, class... Ts> void pr(const T& t, const Ts&... ts) {
		pr(t); pr(ts...);
	}
	template<class T1, class T2> void pr(const pair<T1,T2>& x) {
		pr("{",x.fi,", ",x.se,"}");
	}
	template<class T> void pr(const T& x) {
		pr("{");
		bool fst = 1; for (const auto& a : x) pr(!fst?", ": "",a), fst = 0;
		pr("}");
	}

	void ps() { pr("\n"); }
	template<class T, class... Ts> void ps(const T& t, const Ts&... ts) {
		pr(t); if (sizeof...(ts)) pr(" "); ps(ts...);
	}

	void pc() { pr("]\n"); }
	template<class T, class... Ts> void pc(const T& t, const Ts&... ts) {
		pr(t); if (sizeof...(ts)) pr(", "); pc(ts...);
	}
	#define dbg(x...) pr("[",#x,"] = ["), pc(x);
}

using namespace output;

namespace io {
	void setIn(str s) { freopen(s.c_str(), "r", stdin); }
	void setOut(str s) { freopen(s.c_str(), "w", stdout); }
	void setIO(str s = "") {
		ios_base::sync_with_stdio(0);cin.tie(0);
		if (sz(s)) { setIn(s+".in"), setOut(s+".out"); }
	}
}

using namespace io;

struct mi {
	typedef decay<decltype(MOD)>::type T;
	T val;
	explicit operator T() const { return val; }
	mi() { val = 0; }
	mi(ll v) {
		val = (-MOD <= v && v <= MOD) ? v : v % MOD;
		if (val < 0) val += MOD;
	}
	friend bool operator==(const mi& a, const mi& b) {
		return a.val == b.val;
	}
	friend bool operator!=(const mi& a, const mi& b) {
		return !(a==b);
	}
	friend bool operator<(const mi& a, const mi& b) {
		return a.val < b.val;
	}
	friend bool operator>(const mi& a, const mi& b) {
		return a.val > b.val;
	}
	friend bool operator<=(const mi& a, const mi& b) {
		return a.val <= b.val;
	}
	friend bool operator>=(const mi& a, const mi& b) {
		return a.val >= b.val;
	}
	friend void re(mi& a) { ll x; re(x); a = mi(x); }
	friend void pr(const mi& a) { pr(a.val); }
	friend ostream& operator<<(ostream& os, const mi& a) {
		return os << a.val;
	}
	mi operator-() { return mi(-val); }
	mi& operator+=(const mi& m) {
		if ((val += m.val) >= MOD) val -= MOD;
		return *this;
	}
	mi& operator-=(const mi& m) {
		if ((val -= m.val) < 0) val += MOD;
		return *this;
	}
	mi& operator++() { return *this += 1; }
	mi& operator--() { return *this -= 1; }
	friend mi operator+(mi a, const mi& b) { return a += b; }
	friend mi operator-(mi a, const mi& b) { return a -= b; }
	mi& operator*=(const mi& m) {
		val = (ll)val*m.val%MOD; return *this;
	}
	friend mi pow(mi a, ll p) {
		mi ans = 1; assert(p >= 0);
		for (; p; p/=2, a *= a) if (p&1) ans *= a;
		return ans;
	}
	friend mi inv(const mi& a) {
		assert(!(a==0)); return pow(a, MOD-2);
	}
	mi& operator/=(const mi& m) { return (*this) *= inv(m); }
	friend mi operator*(mi a, const mi& b) { return a *= b; }
	friend mi operator/(mi a, const mi& b) { return a/= b; }
};
typedef pair<mi,mi> pmi;
typedef vector<mi> vmi;
typedef vector<pmi> vpmi;

template<class T> using pqg = pq<T,vector<T>,greater<T>>;
template<class T> T poll(pqg<T>& x) {
	T y = x.top(); x.pop(); return y;
}

template<class T> inline T gcd(T a, T b) { return b ? gcd(b, a % b) : a; }
template<class T> inline T lcm(T a, T b) { return a / gcd(a, b) * b; }
template<class T> inline T ext_gcd(T a, T b, T& x, T& y) { if (!a) { x = 0; y = 1; return b; } T x1, y1; T g = ext_gcd(b % a, a, x1, y1); x = y1 - (b / a) * x1; y = x1; return g; }

string Eval(bool b){return(b?"Yes":"No");}
string eval(bool b){return(b?"yes":"no");}
string EVAL(bool b){return(b?"YES":"NO");}

void precomp() {
	ios_base::sync_with_stdio(0);cin.tie(0);
	setIO("");
}

int h, w, dist[21][21], ans;
char c[21][21];

void dfs(int x, int y, int s) {
	if (x < 0 || y < 0 || x >= h || y >= w || c[x][y] == '#' || dist[x][y] <= s) return;
	dist[x][y] = s;
	F0R(i,4) dfs(x+dx[i],y+dy[i],s+1);
}

void solve(int testCaseNumber) {
	re(h,w);
	F0R(i, h) F0R(j, w) re(c[i][j]);
	F0R(i, h) F0R(j, w) {
		if (c[i][j] == '#') continue;
		F0R(k, h) F0R(l, w) dist[k][l] = 4000;
		dfs(i, j, 0);
		int cur = 0;
		F0R(k, h) F0R(l, w) if ((i != k || j != l) && c[k][l] != '#') ckmax(cur, dist[k][l]);
		ckmax(ans, cur);
	}
	ps(ans);
}

signed main(){

	precomp();
	int t = 1;
//	cin >> t;
	FOR(i,1,t+1) solve(i);	
	return 0;
}



Measured memory_bytes: 256