#include <bits/stdc++.h>
using namespace std;
int n, m, f[402][402];
char s[21][21];
int turn(int x, int y) { return (x - 1) * m + y; }
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= n; ++i) scanf("%s", s[i] + 1);
  int t = n * m;
  for (int i = 1; i <= t; ++i)
    for (int j = 1; j <= t; ++j) f[i][j] = 1e9;
  int u, v;
  for (int i = 1; i <= n; ++i)
    for (int j = 1; j <= m; ++j) {
      u = turn(i, j);
      if (s[i][j] == '#') {
        f[u][u] = 1e9;
        f[u][u + m] = f[u][u - m] = f[u][u - 1] = f[u][u + 1] = 1e9;
        continue;
      }
      f[u][u] = 0;
      if (i > 1 && s[i - 1][j] == '.')
        f[u][u - m] = 1;
      else
        f[u][u - m] = 1e9;
      if (i < n && s[i + 1][j] == '.')
        f[u][u + m] = 1;
      else
        f[u][u + m] = 1e9;
      if (j > 1 && s[i][j - 1] == '.')
        f[u][u - 1] = 1;
      else
        f[u][u - 1] = 1e9;
      if (j < m && s[i][j + 1] == '.')
        f[u][u + 1] = 1;
      else
        f[u][u + 1] = 1e9;
    }
  for (int k = 1; k <= t; ++k)
    for (int i = 1; i <= t; ++i)
      for (int j = 1; j <= t; ++j)
        if (f[i][k] < 1e9 && f[k][j] < 1e9)
          f[i][j] = min(f[i][j], f[i][k] + f[k][j]);
  int ans = 0;
  for (int i = 1; i <= t; ++i)
    for (int j = 1; j <= t; ++j)
      if (f[i][j] < 1e9) ans = max(ans, f[i][j]);
  printf("%d", ans);
}
