# yukicoder My Practice
# author: Leonardone @ NEETSDKASU
############################################################
def gs() gets.chomp end
def gi() gets.to_i end
def gf() gets.to_f end
def gss() gs.split end
def gis() gss.map(&:to_i) end
def gfs() gss.map(&:to_f) end
def nmapf(n,f) n.times.map{ __send__ f } end
def ngs(n) nmapf n,:gs end
def ngi(n) nmapf n,:gi end
def ngss(n) nmapf n,:gss end
def ngis(n) nmapf n,:gis end
def arr2d(h,w,v=0) h.times.map{[v] * w} end
def for2p(hr,wr,&pr) hr.each{|i|wr.each{|j| yield(i,j)}} end
def nsum(n) n * (n + 1) / 2 end
def vcount(d,r=Hash.new(0)) d.inject(r){|m,e| m[e]+=1;m} end
MV4 = [[0, 1], [1, 0], [0, -1], [-1, 0]]
MV8 = MV4 + [[1, 1], [-1, -1], [1, -1], [-1, 1]]
############################################################

H, W = gis
m = ['.' * W] + ngs(H).map{|r| '.' + r + '.' } + ['.' * W]

f = '.'
g = ','

d = 0
loop do
    c = 0
    H.times do |y|
        W.times do |x|
            next if m[y][x] != f
            m[y][x] = '_'
            MV8.each do |dy, dx|
                ey = y + dy
                ex = x + dx
                next if !(0...H).include? ey
                next if !(0...W).include? ex
                next if m[ey][ex] != '#'
                m[ey][ex] = g
                c += 1
            end
        end
    end
    f, g = g, f
    if c == 0
        break
    else
        d += 1
    end
end

puts d