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)*}
    };
}

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)*}
    };

    ($next:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($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, [ $t:tt ]) => {
        {
            let len = read_value!($next, usize);
            (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")
    };
}

#[allow(unused_imports)]
use std::cmp::{min, max};

fn main() {
    input!{
        h: usize,
        w: usize,
        v: [chars; h]
    }
    let mut dist = vec![vec![1000; h * w]; h * w];
    for i in 0..h - 1 {
        for j in 0..w - 1 {
            if v[i][j] == '#' {
                continue;
            }
            dist[i * w + j][i * w + j] = 0;
            if v[i][j + 1] == '.' {
                dist[i * w + j][i * w + j + 1] = 1;
                dist[i * w + j + 1][i * w + j] = 1;
            }
            if v[i + 1][j] == '.' {
                dist[i * w + j][(i + 1) * w + j] = 1;
                dist[(i + 1) * w + j][i * w + j] = 1;
            }
        }
        if v[i][w - 1] == '#' {
            continue;
        }
        dist[i * w + w - 1][i * w + w - 1] = 0;
        if v[i + 1][w - 1] == '.' {
            dist[i * w + w - 1][(i + 1) * w + w - 1] = 1;
            dist[(i + 1) * w + w - 1][i * w + w - 1] = 1;
        }
    }
    for j in 0..w - 1 {
        if v[h - 1][j] == '#' {
            continue;
        }
        dist[(h - 1) * w + j][(h - 1) * w + j] = 0;
        if v[h - 1][j + 1] == '.' {
            dist[(h - 1) * w + j][(h - 1) * w + j + 1] = 1;
            dist[(h - 1) * w + j + 1][(h - 1) * w + j] = 1;
        }
    }
    if v[h - 1][w - 1] == '.' {
        dist[(h - 1) * w + w - 1][(h - 1) * w + w - 1] = 0;
    }
    for k in 0..h * w {
        for i in 0..h * w {
            for j in 0..h * w {
                dist[i][j] = min(dist[i][j], dist[i][k] + dist[k][j]);
            }
        }
    }
    let mut ans = 0;
    for i in 0..h * w {
        for j in i..h * w {
            if dist[i][j] == 1000 {
                continue;
            }
            ans = max(ans, dist[i][j]);
        }
    }
    println!("{}", ans);
}


Measured memory_bytes: 4352