#include <algorithm>
#include<stdio.h>
#include <cstdio>
#include <iostream>
#include <iomanip>
#include <string>
#include <algorithm>
#include <vector>
#include <math.h>
#include <stack>
#include <queue>
#include<map>
#include <set>
#include<math.h>
using namespace std;
#define int long long
#define ll long long
#define double long double
#define rep(i,n) for(ll i=0;i<n;++i)
#define Rep(i,n) for(ll i=1;i<=n;++i)
#define loop(i,m,n) for(ll i=m;i<n;++i)
#define ggr getchar(); getchar();return 0;
#define pie 3.141592653589793238462643383279
#define elif else if
#define P pair<ll,ll>
#define prique priority_queue<ll>
#define ALL(a)  (a).begin(),(a).end()
const int mod = 1000000007;
const int mod2 = 998244353;
int bin(int bina) {
	int ans = 0;
	for (int i = 0; bina > 0; i++)
	{
		ans = ans + (bina % 2) * pow(10, i);
		bina = bina / 2;
	}
	return ans;
}
bool prime(int n) {
	for (ll i = 2; i <= sqrt(n); i++) {
		if (n % i == 0)return false;
	}
	return n != 1;
}
int gcd(int x, int y) {
	if (y == 0)return x;
	return gcd(y, x % y);
}
int lcm(int x, int y) {
	return x * y / gcd(x, y);
}
int kai(int x) {
	if (x == 0)return 1;
	return kai(x - 1) * x % mod;
}
int mod_pow(int x, int y, int m) {
	int res = 1;
	while (y > 0) {
		if (y & 1) {
			res = res * x % m;
		}
		x = x * x % m;
		y >>= 1;
	}
	return res;
}
int nCr(int n, int r) {
	int ans = 1;
	for (ll i = n; i > n - r; --i) {
		ans = ans * i;
	}
	for (ll i = 1; i < r + 1; ++i) {
		ans = ans / i;
	}
	return ans;
}
int per(int n, int r) {
	r = n - r;
	int sum = 1;
	int i;
	for (i = n; i >= r + 1; i--)sum *= i;
	return sum;
}
struct union_find {
	int par[200010], size_[200010];
	void init(int x) {
		rep(i, x) { par[i] = i; size_[i] = 1; }
	}
	int find(int x) {
		if (par[x] == x)return x;
		return par[x] = find(par[x]);
	}
	void unite(int x, int y) {
		x = find(x); y = find(y);
		if (x == y)return;
		if (size_[x] < size_[y]) {
			par[x] = y;
			size_[y] += size_[x];
		}
		else {
			par[y] = x;
			size_[x] += size_[y];
		}
	}
};

int ctoi(const char c) {
	switch (c) {
	case '0': return 0;
	case '1': return 1;
	case '2': return 2;
	case '3': return 3;
	case '4': return 4;
	case '5': return 5;
	case '6': return 6;
	case '7': return 7;
	case '8': return 8;
	case '9': return 9;
	default: return -1;
	}
}
int keta(int a) {
	return to_string(a).size();
}
int h, w;
char c[30][30];
int d[30][30];
int dx[4] = { 1,0,-1,0 }, dy[4] = { 0,1,0,-1 };
int bfs(int a,int b,int q,int e) {
	queue<P>que;
	rep(i, h) {
		rep(j, w) {
			d[i][j] = mod;
		}
	}
	que.push(P(a, b));
	d[a][b] = 0;
	while (que.size()) {
		P p = que.front();que.pop();
		if (p.first == q && p.second == e)break;
		rep(i, 4) {
			int nx = p.first + dx[i], ny = p.second + dy[i];
			if (0 <= nx && nx < h && 0 <= ny && ny < w && c[nx][ny] != '#' && d[nx][ny] == mod) {
				que.push(P(nx, ny));
				d[nx][ny] = d[p.first][p.second] + 1;
			}
		}
	}
	return d[q][e];
}
signed main() {
	cin >> h >> w;
	rep(i, h) {
		rep(j, w) {
			cin >> c[i][j];
		}
	}
	int ans = 0;
	rep(i, h) {
		rep(j, w) {
			rep(k, h) {
				rep(l, w) {
					if (c[i][j] != '#' && c[k][l] != '#') {
						if (bfs(i, j, k, l) > ans)ans = bfs(i, j, k, l);
					}
				}
			}
		}
	}
	cout << ans << endl;
}


Measured memory_bytes: 256