#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <vector>
#include <iomanip>
#include <sstream>
#include <bitset>
#define ALL(x) (x).begin(), (x).end()
#define sz(a) int(a.size())
#define rep(i,x,n) for(int i=x;i<n;i++)
#define repd(i,x,n) for(int i=x;i<=n;i++)
#define pii pair<int,int>
#define pll pair<long long ,long long>
#define gbtb ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
#define MS0(X) memset((X), 0, sizeof((X)))
#define MSC0(X) memset((X), '\0', sizeof((X)))
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define eps 1e-6
#define chu(x) cout<<"["<<#x<<" "<<(x)<<"]"<<endl
#define du3(a,b,c) scanf("%d %d %d",&(a),&(b),&(c))
#define du2(a,b) scanf("%d %d",&(a),&(b))
#define du1(a) scanf("%d",&(a));
using namespace std;
typedef long long ll;
ll gcd(ll a, ll b) {return b ? gcd(b, a % b) : a;}
ll lcm(ll a, ll b) {return a / gcd(a, b) * b;}
ll powmod(ll a, ll b, ll MOD) { if (a == 0ll) {return 0ll;} a %= MOD; ll ans = 1; while (b) {if (b & 1) {ans = ans * a % MOD;} a = a * a % MOD; b >>= 1;} return ans;}
ll poww(ll a, ll b) { if (a == 0ll) {return 0ll;} ll ans = 1; while (b) {if (b & 1) {ans = ans * a ;} a = a * a ; b >>= 1;} return ans;}
void Pv(const vector<int> &V) {int Len = sz(V); for (int i = 0; i < Len; ++i) {printf("%d", V[i] ); if (i != Len - 1) {printf(" ");} else {printf("\n");}}}
void Pvl(const vector<ll> &V) {int Len = sz(V); for (int i = 0; i < Len; ++i) {printf("%lld", V[i] ); if (i != Len - 1) {printf(" ");} else {printf("\n");}}}
inline long long readll() {long long tmp = 0, fh = 1; char c = getchar(); while (c < '0' || c > '9') {if (c == '-') fh = -1; c = getchar();} while (c >= '0' && c <= '9') tmp = tmp * 10 + c - 48, c = getchar(); return tmp * fh;}
inline int readint() {int tmp = 0, fh = 1; char c = getchar(); while (c < '0' || c > '9') {if (c == '-') fh = -1; c = getchar();} while (c >= '0' && c <= '9') tmp = tmp * 10 + c - 48, c = getchar(); return tmp * fh;}
const int maxn = 22;
const int inf = 0x3f3f3f3f;
/*** TEMPLATE CODE * * STARTS HERE ***/
char s[maxn][maxn];
int n, m;
int ans = 0;
bool vis[maxn][maxn];
bool vised[maxn][maxn][maxn][maxn];
struct node
{
    int x, y;
    int step;
    node() {}
    node(int xx, int yy, int ss)
    {
        x = xx;
        y = yy;
        step = ss;
    }
};
queue<node> q;
int aid_x, aid_y;
int now;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
void bfs(int x, int y)
{
    q.push(node(x, y, 0));
    vis[x][y] = 1;
    node temp;
    now = 0;
    while (!q.empty())
    {
        temp = q.front();
        q.pop();
        if (temp.x == aid_x && temp.y == aid_y)
        {
            now = temp.step;
            break;
        }
        for (int i = 0; i < 4; ++i)
        {
            int tx = dx[i] + temp.x;
            int ty = dy[i] + temp.y;
            if (tx >= 1 && tx <= n && ty >= 1 && ty <= m && vis[tx][ty] == 0 && s[tx][ty] == '.')
            {
                vis[tx][ty] = 1;
                q.push(node(tx, ty, temp.step + 1));
            }
        }
    }
    while (!q.empty())
        q.pop();
}
int main()
{
    //freopen("D:\\code\\text\\input.txt","r",stdin);
    //freopen("D:\\code\\text\\output.txt","w",stdout);
    scanf("%d %d", &n, &m);
    repd(i, 1, n)
    {
        scanf("%s", s[i] + 1);
    }
    repd(i, 1, n)
    {
        repd(j, 1, m)
        {
            repd(k, 1, n)
            {
                repd(z, 1, m)
                {
                    if (vised[i][j][k][z] || vised[k][z][i][j])
                    {
                        continue;
                    }
                    vised[i][j][k][z] = 1;
                    vised[k][z][i][j] = 1;
                    if (s[i][j] == '#' || s[k][z] == '#')
                        continue;
                    aid_x = k;
                    aid_y = z;
                    MS0(vis);
                    bfs(i, j);
                    // cout << i << " " << j << " " << k << " " << z << " " << now << endl;
                    ans = max(ans, now);
                }
            }
        }
    }
    printf("%d\n", ans );
    return 0;
}




Measured memory_bytes: 512