#include <bits/stdc++.h>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <cctype>
#include <string>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <algorithm>
#include <functional>
using namespace std; 
#define DEBUG(x) cout << '>' << #x << ':' << x << endl;
#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define FORD(i,a,b) for(int i=(a);i>=(b);i--)
inline bool EQ(double a, double b) { return fabs(a-b) < 1e-9; }
const int INF = 1<<29;
typedef long long int ll;
inline int two(int n) { return 1 << n; }
inline int test(int n, int b) { return (n>>b)&1; }
inline void set_bit(int & n, int b) { n |= two(b); }
inline void unset_bit(int & n, int b) { n &= ~two(b); }
inline int last_bit(int n) { return n & (-n); }
inline int ones(int n) { int res = 0; while(n && ++res) n-=n&(-n); return res; }
template<class T> void chmax(T & a, const T & b) { a = max(a, b); }
template<class T> void chmin(T & a, const T & b) { a = min(a, b); }
const int MOD=1000000007;
#define pp pair<int,int>
bool isPowerOfTwo (ll x)  
{  
    return x && (!(x&(x-1)));  
}
bool isPrime(int n) 
{ 
    if (n <= 1)  return false; 
    if (n <= 3)  return true; 
    if (n%2 == 0 || n%3 == 0) return false; 
  
    for (int i=5; i*i<=n; i=i+6) 
        if (n%i == 0 || n%(i+2) == 0) 
           return false; 
  
    return true; 
}
int highestPowerof2(unsigned int n) 
{ 
    // Invalid input 
    if (n < 1) 
        return 0; 
  
    int res = 1;
    for (int i=0; i<8*sizeof(unsigned int); i++) 
    { 
        int curr = 1 << i; 
        if (curr > n) 
           break; 
  
        res = curr; 
    } 
  
    return res; 
} 
long long binpow(long long a, long long b) {
    long long res = 1;
    while (b > 0) {
        if (b & 1)
            res = res * a;
        a = a * a;
        b >>= 1;
    }
    return res;
}
const int dx[4] = {1,0 ,-1,0};
const int dy[4] = {0,1 ,0,-1};
/*lcm(gcd(N1, M), gcd(N2, M), ..., gcd(Nk, M)) = gcd(lcm(N1, ..., Nk), M)
gcd(lcm(N1, M), lcm(N2, M), ..., lcm(Nk, M)) = lcm(gcd(N1, ..., Nk), M).

If gcd(N1, N2) = 1, then
gcd(N1·N2, M) = gcd(N1, M)·gcd(N2, M)
lcm(N1·N2, M) = lcm(N1, M)·lcm(N2, M)/M.

lcm(M, N, P) · gcd(M, N) · gcd(N, P) · gcd(P, M) = NMP · gcd(N, M, P).
*/
////////////////////////////////////////////////////////////////////

int main()
{
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);  
  cout.tie(NULL);
  int t=1;
  while(t--)
  {
    int h,w,i,j,maxi=0;
    cin>>h>>w;
    vector<string> m(h);
    REP(i,h)
    {
        cin>>m[i];
    }
    for(i=0;i<h;i++)
    {
        for(j=0;j<w;j++)
        {
            if(m[i][j]=='#')continue;
            vector<vector<int>> dist(h,vector<int>(w,-1));
            dist[i][j]=0;
            queue<pp> que;
            que.push({i,j});
            while(!que.empty())
            {
                pp cur=que.front();
                int x=cur.first;
                int y=cur.second;
                que.pop();
                maxi=max(maxi,dist[x][y]);
                for(int dir=0;dir<4;dir++)
                {
                    int next_x = x + dx[dir];
                    int next_y = y + dy[dir];
                    if(next_x>=h || next_y>=w || next_y<0 || next_x<0)continue;
                    if(m[next_x][next_y]=='#')continue;
                    if(dist[next_x][next_y]==-1)
                    {
                        dist[next_x][next_y]=dist[x][y]+1;
                        que.push({next_x,next_y});
                    }
                }
            }
        }
    }
    cout<<maxi;
  }  
  return 0;
}
/* Look for:
* the exact constraints (multiple sets are too slow for n=10^6 :( )
* special cases (n=1?)
* 1LL<<i and not 1<<i
* overflow (int vs ll?)
* array bounds
* if you have no idea just guess the appropriate well-known algo instead of doing nothing :/
*/

Measured memory_bytes: 256