#include <bits/stdc++.h>
using namespace std;
const double PI = 3.14159265358979323846;
const long long int MOD = 1000000000 + 7;
struct UnionFind {
  vector<int> parent;
  UnionFind(int N) : parent(N) {
    for (int i = 0; i < N; i++) {
      parent[i] = -1;
    }
  }
  int root(int i) {
    if (parent[i] < 0) {
      return i;
    }
    return (parent[i] = root(parent[i]));
  }
  bool unite(int x, int y) {
    int rx = root(x);
    int ry = root(y);
    if (rx != ry) {
      parent[ry] += parent[rx];
      parent[rx] = ry;
      return true;
    } else {
      return false;
    }
  }
  bool same(int x, int y) { return root(x) == root(y); }
  int treeSize(int x) { return -parent[root(x)]; }
};
long long int modpow(long long int base, long long int pow, long long int mod) {
  if (pow == 1) {
    return base;
  } else if (pow == 0) {
    return 1;
  }
  if (pow % 2 == 0) {
    auto temp = modpow(base, pow / 2, mod);
    return (temp * temp) % mod;
  } else {
    return base * modpow(base, pow - 1, mod) % mod;
  }
}
long long int moddiv(long long int X, long long int Y, long long int mod) {
  auto fermatDiv = modpow(Y, mod - 2, mod);
  return (X * fermatDiv) % mod;
}
bool IsPrime(long long N) {
  if (N == 1) {
    return false;
  }
  for (long long i = 2; i * i <= N; i++) {
    if (N % i == 0) {
      return false;
    }
  }
  return true;
}
bool IsPE(long long z) {
  for (long long i = 2; i <= z; i++) {
    if (z % i == 0 && IsPrime(i)) {
      long long test = i;
      while (test < z) {
        test *= i;
      }
      if (test == z) {
        return true;
      }
    }
  }
  return false;
}
long long solve(long long N) {
  long long answer = 0;
  long long divider = 2;
  while (N >= divider) {
    if (N % divider == 0 && IsPE(divider)) {
      answer++;
      N /= divider;
    }
    divider++;
  }
  return answer;
}
vector<pair<long long, long long>> prime_factorize(long long N) {
  vector<pair<long long, long long>> res;
  for (long long a = 2; a * a <= N; ++a) {
    if (N % a != 0) continue;
    long long ex = 0;
    while (N % a == 0) {
      ++ex;
      N /= a;
    }
    res.push_back({a, ex});
  }
  if (N != 1) res.push_back({N, 1});
  return res;
}
int H, W;
void Update(vector<vector<int>>& nodes, int from, int to) {
  nodes[from][to] = 1;
  nodes[to][from] = 1;
  for (int i = 0; i < H * W; i++) {
    if (nodes[to][i] != -1 &&
        (nodes[from][i] > nodes[to][i] + 1 || nodes[from][i] == -1)) {
      nodes[from][i] = nodes[to][i] + 1;
      nodes[i][from] = nodes[from][i];
    }
  }
  for (int i = 0; i < H * W; i++) {
    if (nodes[from][i] != -1 &&
        (nodes[to][i] == -1 || nodes[from][i] > nodes[to][i] + 1)) {
      nodes[to][i] = nodes[from][i] + 1;
      nodes[i][to] = nodes[to][i];
      for (int j = 0; j < H * W; j++) {
        if (nodes[to][j] != -1 &&
            (nodes[j][i] == -1 ||
             nodes[j][i] > nodes[j][to] + 1 + nodes[from][i])) {
          nodes[j][i] = nodes[j][to] + 1 + nodes[from][i];
          nodes[i][j] = nodes[j][i];
        }
      }
    }
  }
}
int main() {
  cin >> H >> W;
  vector<vector<int>> nodes(H * W);
  vector<bool> maze(H * W);
  for (int i = 0; i < H * W; i++) {
    maze[i] = false;
    nodes[i] = vector<int>(H * W);
    for (int j = 0; j < H * W; j++) {
      if (i == j) {
        nodes[i][j] = 0;
      } else {
        nodes[i][j] = -1;
      }
    }
  }
  for (int i = 0; i < H * W; i++) {
    char c;
    cin >> c;
    if (c == '.') {
      maze[i] = true;
      if (i % W != 0 && maze[i - 1]) {
        Update(nodes, i, i - 1);
      }
      if ((i + 1) % W != 0 && maze[i + 1]) {
        Update(nodes, i, i + 1);
      }
      if (i - W >= 0 && maze[i - W]) {
        Update(nodes, i, i - W);
      }
      if (i + W < H * W && maze[i + W]) {
        Update(nodes, i, i + W);
      }
    }
  }
  int max = -1;
  for (int i = 0; i < H * W; i++) {
    for (int j = 0; j < H * W; j++) {
      if (nodes[i][j] > max) {
        max = nodes[i][j];
      }
    }
  }
  cout << max << endl;
  return 0;
}
