#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
 
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
 
#define start_routine() int begtime = clock();
#define end_routine() int endtime = clock(); cerr << endl << "Time elapsed: " << (endtime - begtime)*1000/CLOCKS_PER_SEC << " ms"; return 0
#define speed() cin.tie(0), cout.tie(0), ios_base::sync_with_stdio(false)
// #define exit(a, b) return cout << a, b;
 
// debugger
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
    cout << name << " : " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
    const char* comma = strchr(names + 1, ',');cout.write(names, comma - names) << " : " << arg1<<" | ";__f(comma+1, args...);
}
#define debug(stuff) cout << #stuff << ": " << stuff <<endl
#define debugc(stuff) cout << #stuff << ": "; for(auto x: stuff) cout << x << " "; cout << endl;
// 
 
#define loop(i,a,b) for(ll i=a;i<b;i++)
#define all(v) v.begin(), v.end() 
 
#define print(stuff) cout << stuff << endl
#define printc(stuff) for(auto x: stuff) cout << x << " "; cout << endl;
#define printPrec(stuff) cout << fixed << setprecision(15) << stuff << endl;
#define len length
#define ret0 return 0
#define ret return 
 
 
#define ll long long
#define ld long double
#define fi first
#define endl '\n'
#define se second
#define pb push_back
#define mp make_pair
#define lb lower_bound
#define ub upper_bound
#define fill(ar, x) memset(ar, x, sizeof ar)
 
#define vl vector<ll> 
#define sl set<ll>
#define pll pair<ll, ll>
#define mll map<ll, ll> 
#define pq priority_queue<ll>
 
// typedef tree<ll,null_type,less<ll>,rb_tree_tag,
// tree_order_statistics_node_update> indexed_set;
 
#define inf (long long int) 1e18
#define eps 0.000001
#define mod 1000000007
#define mod1 998244353
#define MAXN (ll)2e5+5
#define N (1ll<<10)
#define pi 3.141592653589793238462643

string s[25];

bool in(ll x, ll y, ll h, ll w){
    if(x >= 0 && x < h && y >= 0 && y < w) return true;
    return false;
}

ll f(ll x, ll y, ll h, ll w){
    ll a[h][w];
    loop(i,0,h){
        loop(j,0,w){
            a[i][j] = inf;
        }
    }
    a[x][y] = 0;
    priority_queue<pair<ll, pll>> q;
    q.push(mp(0, mp(x, y)));
    bool vis[h][w];
    loop(i,0,h) loop(j,0,w) vis[i][j] = false;

    while(!q.empty()){
        ll weight = -q.top().fi;
        pll rr = q.top().se;
        q.pop();
        // trace(rr.fi, rr.se);
        if(vis[rr.fi][rr.se]) continue;
        vis[rr.fi][rr.se] = true;

        if(in(rr.fi-1, rr.se, h, w) && s[rr.fi-1][rr.se] == '.'){
            if(a[rr.fi-1][rr.se] > weight+1){
                a[rr.fi-1][rr.se] = weight+1;
                q.push(mp(-(weight+1), mp(rr.fi-1, rr.se)));
            }
        }
        if(in(rr.fi+1, rr.se, h, w) && s[rr.fi+1][rr.se] == '.'){
            if(a[rr.fi+1][rr.se] > weight+1){
                a[rr.fi+1][rr.se] = weight+1;
                q.push(mp(-(weight+1), mp(rr.fi+1, rr.se)));
            }
        }
        if(in(rr.fi, rr.se-1, h, w) && s[rr.fi][rr.se-1] == '.'){
            if(a[rr.fi][rr.se-1] > weight+1){
                a[rr.fi][rr.se-1] = weight+1;
                q.push(mp(-(weight+1), mp(rr.fi, rr.se-1)));
            }
        }
        if(in(rr.fi, rr.se+1, h, w) && s[rr.fi][rr.se+1] == '.'){
            if(a[rr.fi][rr.se+1] > weight+1){
                a[rr.fi][rr.se+1] = weight+1;
                q.push(mp(-(weight+1), mp(rr.fi, rr.se+1)));
            }
        }
    }
    ll mx = -inf;
    loop(i,0,h){
        loop(j,0,w){
            if(s[i][j] == '.')
            mx = max(mx, a[i][j]);
        }
    }
    // trace(mx);
    return mx;
}
void solve(){
    ll h, w;
    cin>>h>>w;
    loop(i,0,h){
        cin>>s[i];
    }

    ll ans = -inf;
    loop(i,0,h){
        loop(j,0,w){
            // trace(i, j);
            if(s[i][j] == '.'){
                ans = max(ans, f(i, j, h, w));
            }
        }
    }
    print(ans);

}
 
int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

 
    ll t = 1;
    // cin>>t;
    while(t--){
        solve();
    }
 
}


Measured memory_bytes: 256