using namespace std;

#include<bits/stdc++.h>

typedef long long ll;
typedef pair<int, int> ii;
typedef pair<ll, ll> pll;
typedef vector<ii> vii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vll;
typedef vector<vll> vvll;

#define getBit(x,i) (x&(1<<i))
#define setBit(x,i) (x|(1<<i))
#define mem(A,N) memset(A,N,sizeof(A))
#define pb push_back
#define pf push_front
#define ppb pop_back
#define ppf pop_front
#define mp make_pair
#define rep(i, x, y)  for(int i = x; i < y; ++i)
#define Rep(i, x, y)  for(int i = x; i <= y; ++i)
#define srep(i, x, y) for(int i = x; i > y; --i)
#define sRep(i, x, y) for(int i = x; i >= y; --i)
#define All(v) v.begin(),v.end()
#define sortA(a) sort(a, a+n)
#define sortArr(a, i, n) sort(a+i, a+n)
#define SortArr(a, i, n) sort(a+i, a+n+1)
#define Sort(x) sort(x.begin(), x.end())
#define f1(i,n) for (int i=1; i<=n; i++)
#define f0(i,n) for (int i=0; i<n; i++)
#define sz(x) ((int)(x).size())
#define len(x) ((int)(x).length())
#define fi first
#define se second
#define concatenate(a, b) std::copy(b.begin(), b.end(), std::back_inserter(a));
// #define concatenate(a, b) a.insert(b.end(), b.begin(), b.end());
// std::move(b.begin(), b.end(), std::back_inserter(a)); "Undefined behaviour if a actually is b (which is OK if you know that can never happen - but worth being aware of in general purpose code)"
#define makeUnique(v) v.erase(unique(v.begin(), v.end()), v.end())
#define inRange(x, a, b) (a <= x && x < b)

#define linf 0x7FFFFFFFFFFFFFFF
#define inf 0x7FFFFFFF
#define mod 1000000007
#define eps 1e-9
#define Pi 3.14159265358979323846

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define write(x) cerr << #x << " = " << (x) << ", ";
#define writeI0 cerr << "i = " << 0 << ", ";
#define dumpI0 cerr << "i = " << 0 << endl;
#define printSet(s) for(auto it = s.begin(); it!=s.end(); it++) (it==s.begin())? cout << *it : cout << ' ' << *it; cout << endl;
#define printMap(m) for(auto it = m.begin(); it!=m.end(); it++) (it==m.begin())? cout << '(' << it->first << ", " << it->second << ')' : cout << " (" << it->first << ", " << it->second << ')'; cout << endl;
#define printPairSet(s) for(auto it = s.begin(); it!=s.end(); it++) (it==s.begin())? cout << '(' << it->first << ", " << it->second << ')' : cout << " (" << it->first << ", " << it->second << ')'; cout << endl;
#define printVector(x) for(auto it = x.begin(); it!=x.end(); it++) (it==x.begin())? cout << *it : cout << ' ' << *it; cout << endl;
#define printArray(a,x,y) for(auto it = a+x; it != a+y; it++) (it==a+x)? cout << *it : cout << ' ' << *it; cout << endl;
#define PrintArray(a,x,y) for(auto it = a+x; it != a+y+1; it++) (it==a+x)? cout << *it : cout << ' ' << *it; cout << endl;

// ***************** End Of Template ********************

int h, w;
char a[20][20];
int dx[] = {1, 0, -1, 0};
int dy[] = {0, -1, 0, 1};
int d[21][21];
queue<ii> q;
bool v[21][21];

int main() {
	ios_base::sync_with_stdio(false); cin.tie(NULL);
	cin >> h >> w; char c;
	rep(i, 0, h) {
		rep(j, 0, w) {
			cin >> a[i][j];
		}
	}



	int sol = 0;

	rep(i, 0, h) {
		rep(j, 0, w) {
			rep(y, 0, h) {
				rep(x, 0, w) {
					d[y][x] = 1000000;
					v[y][x] = false;
				}
			}


			if (a[i][j] == '.') {
				int mxDist = 0;
				d[i][j] = 0;
				q.push(mp(i, j));
				v[i][j] = true;
				while (!q.empty()) {
					ii z = q.front(); q.pop();
					int y = z.fi, x = z.se;
					int dist = d[y][x];
					rep(p, 0, 4) {
						int t1, t2;
						t1 = y+dy[p];
						t2 = x+dx[p];
						if (t1 >= 0 && t1 < h && t2 >= 0 && t2 < w) {
							if (a[t1][t2] == '.' && !v[t1][t2]) {
								d[t1][t2] = 1 + dist;
								mxDist = max(mxDist, 1 + dist);
								q.push(mp(t1, t2));
								v[t1][t2] = true;
							}
						}
					}
				}
				sol = max(sol, mxDist);
			}
		}
	}

	cout << sol;





	return 0;
}

Measured memory_bytes: 256