--https://www.lua.org/pil/11.4.html
local List = {}
function List.new ()
  return {first = 0, last = -1}
end

function List.pushleft (list, value)
  local first = list.first - 1
  list.first = first
  list[first] = value
end

function List.pushright (list, value)
  local last = list.last + 1
  list.last = last
  list[last] = value
end

function List.popleft (list)
  local first = list.first
  if first > list.last then return false end
  local value = list[first]
  list[first] = nil
  list.first = first + 1
  return value
end

function List.popright (list)
  local last = list.last
  if list.first > last then return false end
  local value = list[last]
  list[last] = nil
  list.last = last - 1
  return value
end

----------

local h,w=io.read("n","n","l")
local a={}
for i=1,h do
    local input=io.read()
    a[i]={}
    for j=1,w do
        a[i][j]=input:byte(j)
    end
end

local dist={}
local que=List.new()
for i=1,h do
    dist[i]={}
    for j=1,w do
        if a[i][j]==35 then
            dist[i][j]=0
            List.pushright(que,{i,j})
        else
            dist[i][j]=-1
        end
    end
end

local di={1,0,-1,0}
local dj={0,1,0,-1}
while true do
    local pop=List.popleft(que)
    if not pop then
        break
    end
    local i=pop[1]
    local j=pop[2]
    for k=1,4 do
        local ni=i+di[k]
        local nj=j+dj[k]
        if dist[ni] and dist[ni][nj] and dist[ni][nj]==-1 then
            dist[ni][nj]=dist[i][j]+1
            List.pushright(que,{ni,nj})
        end 
    end
end

local max=0
for i=1,h do
    for j=1,w do
        max=math.max(max,dist[i][j])
    end
end
print(max)