package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

func getScanner(fp *os.File) *bufio.Scanner {
	scanner := bufio.NewScanner(fp)
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 1000005), 1000005)
	return scanner
}
func getNextString(scanner *bufio.Scanner) string {
	scanner.Scan()
	return scanner.Text()
}
func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}
func getNextInt64(scanner *bufio.Scanner) int64 {
	i, _ := strconv.ParseInt(getNextString(scanner), 10, 64)
	return i
}
func getNextUint64(scanner *bufio.Scanner) uint64 {
	i, _ := strconv.ParseUint(getNextString(scanner), 10, 64)
	return i
}
func getNextFloat64(scanner *bufio.Scanner) float64 {
	i, _ := strconv.ParseFloat(getNextString(scanner), 64)
	return i
}
func main() {
	fp := os.Stdin
	wfp := os.Stdout
	cnt := 0
	if os.Getenv("MASPY") == "ますピ" {
		fp, _ = os.Open(os.Getenv("BEET_THE_HARMONY_OF_PERFECT"))
		cnt = 3
	}
	if os.Getenv("MASPYPY") == "ますピッピ" {
		wfp, _ = os.Create(os.Getenv("NGTKANA_IS_GENIUS10"))
	}
	scanner := getScanner(fp)
	writer := bufio.NewWriter(wfp)
	solve(scanner, writer)
	for i := 0; i < cnt; i++ {
		fmt.Fprintln(writer, "-----------------------------------")
		solve(scanner, writer)
	}
	writer.Flush()
}
func solve(scanner *bufio.Scanner, writer *bufio.Writer) {
	w := getNextInt(scanner)
	h := getNextInt(scanner)
	ss := make([]string, h)
	for i := 0; i < h; i++ {
		ss[i] = getNextString(scanner)
	}
	visit := makeGrid(h, w)
	q := make([]queue, 0)
	empties(h, w, ss, visit)
	for i := 0; i < h; i++ {
		for j := 0; j < w; j++ {
			if visit[i][j] == 1 {
				q = append(q, newQueue(i, j, 1))
			}
		}
	}
	dy := [4]int{-1, 0, 0, 1}
	dx := [4]int{0, -1, 1, 0}
	for len(q) > 0 {
		p := q[0]
		q = q[1:]
		for i := 0; i < 4; i++ {
			yy := p.y + dy[i]
			xx := p.x + dx[i]
			if yy < 0 || yy >= h {
				continue
			}
			if xx < 0 || xx >= w {
				continue
			}
			if visit[yy][xx] > 0 {
				continue
			}
			visit[yy][xx] = p.c + 1
			q = append(q, newQueue(yy, xx, p.c+1))
		}
	}
	ans := 0
	for i := 0; i < h; i++ {
		for j := 0; j < w; j++ {
			if ss[i][j] == '#' {
				continue
			}
			if visit[i][j] == 1 {
				continue
			}
			if ans == 0 || ans > visit[i][j] {
				ans = visit[i][j]
			}
		}
	}
	fmt.Fprintln(writer, ans-2)
}

func makeGrid(h, w int) [][]int {
	index := make([][]int, h, h)
	data := make([]int, h*w, h*w)
	for i := 0; i < h; i++ {
		index[i] = data[i*w : (i+1)*w]
	}
	return index
}

func empties(h, w int, ss []string, visit [][]int) {
	for i := 0; i < h; i++ {
		for j := 0; j < w; j++ {
			if ss[i][j] == '.' {
				rec(h, w, i, j, ss, visit)
				return
			}
		}
	}
}
func rec(h, w, y, x int, ss []string, visit [][]int) {
	if y < 0 || y >= h {
		return
	}
	if x < 0 || x >= w {
		return
	}
	if ss[y][x] != '.' {
		return
	}
	if visit[y][x] != 0 {
		return
	}
	visit[y][x] = 1
	dy := [4]int{-1, 0, 0, 1}
	dx := [4]int{0, -1, 1, 0}
	for i := 0; i < 4; i++ {
		yy := y + dy[i]
		xx := x + dx[i]
		rec(h, w, yy, xx, ss, visit)
	}
}
func newQueue(y, x, c int) queue {
	return queue{y: y, x: x, c: c}
}

type queue struct {
	y, x, c int
}