import java.util.Arrays;

public class Main {
  private static void solve() {
    int h = ni();
    int w = ni();


    char[][] map = new char[h + 2][w + 2];
    for (int i = 0; i < h; i++) {
      char[] l = ns();
      for (int j = 0; j < w; j++) {
        map[i + 1][j + 1] = l[j];
      }
    }

    int n = (h + 2) * (w + 2);
    int[][] d = new int[n][n];
    for (int[] v : d)
      Arrays.fill(v, Integer.MAX_VALUE / 100);
    for (int i = 0; i < n; i++) {
      int y = i / (w + 2);
      int x = i % (w + 2);
      if (map[y][x] != '.')
        continue;
      d[i][i] = 0;
    }

    int[][] dir = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
    for (int i = 0; i < n; i++) {
      int y = i / (w + 2);
      int x = i % (w + 2);
      if (map[y][x] != '.')
        continue;

      for (int[] dd : dir) {
        if (map[y + dd[0]][x + dd[1]] == '.') {
          int ni = (y + dd[0]) * (w + 2) + (x + dd[1]);
          d[i][ni] = d[ni][i] = 1;
        }
      }
    }

    for (int k = 0; k < n; k++) {
      for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
          if (d[i][j] > d[i][k] + d[k][j]) {
            d[i][j] = d[i][k] + d[k][j];
          }
        }
      }
    }

    int ret = 0;
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < n; j++) {
        if (d[i][j] >= Integer.MAX_VALUE / 100)
          continue;
        ret = Math.max(d[i][j], ret);
      }
    }
    System.out.println(ret);
  }



  public static void main(String[] args) {
    new Thread(null, new Runnable() {
      @Override
      public void run() {
        long start = System.currentTimeMillis();
        String debug = args.length > 0 ? args[0] : null;
        if (debug != null) {
          try {
            is = java.nio.file.Files.newInputStream(java.nio.file.Paths.get(debug));
          } catch (Exception e) {
            throw new RuntimeException(e);
          }
        }
        reader = new java.io.BufferedReader(new java.io.InputStreamReader(is), 32768);
        solve();
        out.flush();
        tr((System.currentTimeMillis() - start) + "ms");
      }
    }, "", 64000000).start();
  }

  private static java.io.InputStream is = System.in;
  private static java.io.PrintWriter out = new java.io.PrintWriter(System.out);
  private static java.util.StringTokenizer tokenizer = null;
  private static java.io.BufferedReader reader;

  public static String next() {
    while (tokenizer == null || !tokenizer.hasMoreTokens()) {
      try {
        tokenizer = new java.util.StringTokenizer(reader.readLine());
      } catch (Exception e) {
        throw new RuntimeException(e);
      }
    }
    return tokenizer.nextToken();
  }

  private static double nd() {
    return Double.parseDouble(next());
  }

  private static long nl() {
    return Long.parseLong(next());
  }

  private static int[] na(int n) {
    int[] a = new int[n];
    for (int i = 0; i < n; i++)
      a[i] = ni();
    return a;
  }

  private static char[] ns() {
    return next().toCharArray();
  }

  private static long[] nal(int n) {
    long[] a = new long[n];
    for (int i = 0; i < n; i++)
      a[i] = nl();
    return a;
  }

  private static int[][] ntable(int n, int m) {
    int[][] table = new int[n][m];
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
        table[i][j] = ni();
      }
    }
    return table;
  }

  private static int[][] nlist(int n, int m) {
    int[][] table = new int[m][n];
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
        table[j][i] = ni();
      }
    }
    return table;
  }

  private static int ni() {
    return Integer.parseInt(next());
  }

  private static void tr(Object... o) {
    if (is != System.in)
      System.out.println(java.util.Arrays.deepToString(o));
  }
}


Measured memory_bytes: 25784