#[allow(unused_imports)]
use std::cmp;
use std::io::Write;

// Input macro from https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
  (source = $s:expr, $($r:tt)*) => {
    let mut iter = $s.split_whitespace();
    input_inner!{iter, $($r)*}
  };
  ($($r:tt)*) => {
    let s = {
      use std::io::Read;
      let mut s = String::new();
      std::io::stdin().read_to_string(&mut s).unwrap();
      s
    };
    let mut iter = s.split_whitespace();
    input_inner!{iter, $($r)*}
  };
}

macro_rules! input_inner {
  ($iter:expr) => {};
  ($iter:expr, ) => {};

  ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
    let $var = read_value!($iter, $t);
    input_inner!{$iter $($r)*}
  };
}

macro_rules! read_value {
  ($iter:expr, ( $($t:tt),* )) => {
    ( $(read_value!($iter, $t)),* )
  };

  ($iter:expr, [ $t:tt ; $len:expr ]) => {
    (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
  };

  ($iter:expr, chars) => {
    read_value!($iter, String).chars().collect::<Vec<char>>()
  };

  ($iter:expr, usize1) => {
    read_value!($iter, usize) - 1
  };

  ($iter:expr, [ $t:tt ]) => {{
    let len = read_value!($iter, usize);
    (0..len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
  }};

  ($iter:expr, $t:ty) => {
    $iter.next().unwrap().parse::<$t>().expect("Parse error")
  };
}

const INF: i32 = 1 << 30;
use std::collections::VecDeque;

fn bfs(dist: &mut Vec<Vec<i32>>, fld: &Vec<Vec<char>>, si: usize, sj: usize, h: i32, w: i32) {
  let mut q = VecDeque::new();
  q.push_back((si, sj));
  while let Some((i, j)) = q.pop_front() {
    let ii = i as i32;
    let ij = j as i32;
    for &(di, dj) in &[(0, 1), (1, 0), (0, -1), (-1, 0)] {
      let ni = ii + di;
      let nj = ij + dj;
      if 0 <= ni && ni < h && 0 <= nj && nj < w {
        let ni = ni as usize;
        let nj = nj as usize;
        if fld[ni][nj] == '.' {
          if dist[i][j] + 1 < dist[ni][nj] {
            dist[ni][nj] = dist[i][j] + 1;
            q.push_back((ni, nj));
          }
        }
      }
    }
  }
}

fn main() {
  let out = std::io::stdout();
  let mut out = std::io::BufWriter::new(out.lock());
  macro_rules! puts {
    ($($format:tt)*) => (write!(out,$($format)*).unwrap());
  }
  input! {
    h: usize,
    w: usize,
    fld: [chars; h],
  }
  let mut temp_start = (0, 0);
  for i in 0 .. h {
    for j in 0 .. w {
      if fld[i][j] == '.' {
        temp_start = (i, j);
        break;
      }
    }
  }
  let (ti, tj) = temp_start;
  let mut dist = vec![vec![INF; w]; h];
  dist[ti][tj] = 0;
  bfs(&mut dist, &fld, ti, tj, h as i32, w as i32);
  let mut mi = 0;
  let mut mj = 0;
  for i in 0 .. h {
    for j in 0 .. w {
      if fld[i][j] == '.' && dist[i][j] > dist[mi][mj] {
        mi = i;
        mj = j;
      }
    }
  }
  let mut dist2 = vec![vec![INF; w]; h];
  dist2[mi][mj] = 0;
  bfs(&mut dist2, &fld, mi, mj, h as i32, w as i32);
  let mut ans = 0;
  for i in 0 .. h {
    for j in 0 .. w {
      if fld[i][j] == '.' {
        ans = cmp::max(ans, dist2[i][j]);
      }
    }
  }

  puts!("{}\n", ans);
}
