#![allow(unused)]

//入力マクロここから
//https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules ! input_inner { ( $ next : expr ) => { } ; ( $ next : expr , ) => { } ; ( $ next : expr , $ var : ident : $ t : tt $ ( $ r : tt ) * ) => { let $ var = read_value ! ( $ next , $ t ) ; input_inner ! { $ next $ ( $ r ) * } } ; }
macro_rules ! read_value { ( $ next : expr , ( $ ( $ t : tt ) ,* ) ) => { ( $ ( read_value ! ( $ next , $ t ) ) ,* ) } ; ( $ next : expr , [ $ t : tt ; $ len : expr ] ) => { ( 0 ..$ len ) . map ( | _ | read_value ! ( $ next , $ t ) ) . collect ::< Vec < _ >> ( ) } ; ( $ next : expr , chars ) => { read_value ! ( $ next , String ) . chars ( ) . collect ::< Vec < char >> ( ) } ; ( $ next : expr , usize1 ) => { read_value ! ( $ next , usize ) - 1 } ; ( $ next : expr , $ t : ty ) => { $ next ( ) . parse ::<$ t > ( ) . expect ( "Parse error" ) } ; }
macro_rules ! input { ( source = $ s : expr , $ ( $ r : tt ) * ) => { let mut iter = $ s . split_whitespace ( ) ; let mut next = || { iter . next ( ) . unwrap ( ) } ; input_inner ! { next , $ ( $ r ) * } } ; ( $ ( $ r : tt ) * ) => { let stdin = std :: io :: stdin ( ) ; let mut bytes = std :: io :: Read :: bytes ( std :: io :: BufReader :: new ( stdin . lock ( ) ) ) ; let mut next = move || -> String { bytes . by_ref ( ) . map ( | r | r . unwrap ( ) as char ) . skip_while ( | c | c . is_whitespace ( ) ) . take_while ( | c |! c . is_whitespace ( ) ) . collect ( ) } ; input_inner ! { next , $ ( $ r ) * } } ; }
//入力マクロここまで

//input
//use std::io::Read;

//collections
//use std::collections::BTreeMap;
//use std::collections::BTreeSet;
//use std::collections::HashMap;
//use std::collections::HashSet;
use std::cmp::*;
use std::collections::VecDeque;

fn bfs(st: usize, gr: &Vec<Vec<usize>>, dep: &mut Vec<usize>) -> usize {
    let mut que = VecDeque::new();
    let mut now;
    let mut seen = vec![false; gr.len()];

    que.push_back(st);
    seen[st] = true;

    while !que.is_empty() {
        now = que.pop_front().unwrap();

        for e in gr[now].iter() {
            if !seen[*e] {
                que.push_back(*e);
                dep[*e] = dep[now] + 1;
                seen[*e] = true;
            }
        }
    }

    *dep.iter().max().unwrap()
}

fn main() {
    input! {
        h: usize,
        w: usize,
        s: [chars;h]
    }

    let mut gr = vec![vec![]; h * w];

    for i in 0..h {
        for j in 0..w {
            if s[i][j] == '#' {
                continue;
            }
            if i != 0 {
                if s[i - 1][j] == '.' {
                    gr[w * i + j].push(w * (i - 1) + j);
                }
            }
            if j != w - 1 {
                if s[i][j + 1] == '.' {
                    gr[w * i + j].push(w * i + j + 1);
                }
            }
            if j != 0 {
                if s[i][j - 1] == '.' {
                    gr[w * i + j].push(w * i + j - 1);
                }
            }
            if i != h - 1 {
                if s[i + 1][j] == '.' {
                    gr[w * i + j].push(w * (i + 1) + j);
                }
            }
        }
    }

    let mut dep = vec![0; h * w];

    for i in 0..h * w {
        dep[i] = bfs(i, &gr, &mut vec![0; h * w]);
    }

    println!("{}", dep.iter().max().unwrap());
}


Measured memory_bytes: 4352