#include <iostream>
#include <map>
#include <vector>
#include <algorithm>
#include <string>
#include <queue>
#include <iomanip>
#include <set>
#include <unordered_map>

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

using namespace std;

#define rep(i,n) for(int i=0,i##_len=(n);i<i##_len;++i)
#define repx(i,s,d,e) for(int i=s,i##_end=(e);i<i##_end;i+=d)
#define repxr(i,s,d,e) for(int i=s,i##_end=(e);i>i##_end;i+=d)
#define rept(n) for(int ___i___=0,i##_len=(n);___i___<i##_len;++___i___)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define ALL(x) (x).begin(),(x).end()
#define SZ(x) ((int)(x).size())
#define ZERO(a) memset(a,0,sizeof(a))
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(v.begin(),v.end()),v.end());
#define CEIL(x, y) (((x) + (y) - 1) / (y))

// vector
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
    for (T &x : vec) is >> x;
    return is;
}
// pair
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}
// vector
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &vec) {
    os << "{";
    for (int i = 0; i < vec.size(); i++) {
        os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
    }
    os << "}";
    return os;
}
// map
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
    os << "{";
    repi(itr, map_var) {
        os << *itr;
        itr++;
        if (itr != map_var.end()) os << ", ";
        itr--;
    }
    os << "}";
    return os;
}
// set
template <typename T>
ostream &operator<<(ostream &os, set<T> &set_var) {
    os << "{";
    repi(itr, set_var) {
        os << *itr;
        itr++;
        if (itr != set_var.end()) os << ", ";
        itr--;
    }
    os << "}";
    return os;
}
#define DUMPOUT cout
void dump_func() {
    DUMPOUT << endl;
}
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) {
        DUMPOUT << ", ";
    }
    dump_func(std::move(tail)...);
}
#ifdef LOCAL_
#define dump(...)                                                              \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                            \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
            << endl                                                            \
            << "    ",                                                         \
        dump_func(__VA_ARGS__)
#else
#define dump(...)
#endif

typedef long long int lli;
typedef pair<int, int> ii;
typedef priority_queue<int, vector<int>, greater<int> > heapq;
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};
struct aaa{
    aaa(){
        cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(20);
    };
}aaaaaaa;

// #define int long long int
// struct S{
//     bool operator<(const S& x) const {
//         return ;
//     }
// };
int gW = 0;
inline int flat(int i, int j) {
    return i * gW + j;
}
signed main() {
    int H, W; cin >> H >> W; gW = W;
    int cell[20][20]; ZERO(cell);
    rep(i, H) {
        string S; cin >> S;
        rep (j, W) {
            cell[i][j] = S[j] == '#' ? 0 : 1;
        }
    }

    int dist[400][400];
    int len = H * W;
    rep(i, len) {
        rep(j, len) {
            dist[i][j] = i == j ? 0 : 1e9;
        }
    }

    rep(i, H) {
        rep(j, W) {
            if (!cell[i][j]) continue;
            if (i < H - 1 && cell[i + 1][j]) {
                dist[flat(i, j)][flat(i + 1, j)] = 1;
                dist[flat(i + 1, j)][flat(i, j)] = 1;
            }
            if (j < W - 1 && cell[i][j + 1]) {
                dist[flat(i, j)][flat(i, j + 1)] = 1;
                dist[flat(i, j + 1)][flat(i, j)] = 1;
            }
        }
    }

    rep(k, len) {
        rep (i, len) {
            rep (j, len) {
                if (dist[i][k] + dist[k][j] <= dist[i][j]) {
                    dist[i][j] = dist[i][k] + dist[k][j];
                }
            }
        }
    }

    int ans = 0;
    rep(i, len) {
        rep(j, len) {
                dump(i, j, dist[i][j]);
            if (dist[i][j] != 1e9) {
                ans = max(ans, dist[i][j]);
            }
        }
    }
    cout << ans << endl;
}


Measured memory_bytes: 896