--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 maze={}
for i=1,h do
    local s=io.read()
    maze[i]={}
    for j=1,w do
        maze[i][j]=s:sub(j,j)
    end
end

local dx={1,0,-1,0}
local dy={0,1,0,-1}

local function bfs(x,y)
    local dist={}
    for i=1,h do
        dist[i]={}
        for j=1,w do
            dist[i][j]=-1
        end
    end
    local que=List.new()
    
    local result=0
    List.pushright(que,{x,y})
    dist[x][y]=0
    while true do
        local q=List.popleft(que)
        if not q then
            break
        end
        x,y=q[1],q[2]
        result=math.max(result,dist[x][y])
        for i=1,4 do
            local nx,ny=x+dx[i],y+dy[i]
            local checker=(nx<1 or nx>h or ny<1 or ny>w)
            if not checker and maze[nx][ny]~="#" and dist[nx][ny]==-1 then
                dist[nx][ny]=dist[x][y]+1
                List.pushright(que,{nx,ny})
            end
        end
    end

    return result
end

local result=0
for i=1,h do
    for j=1,w do
        if maze[i][j]~="#" then
            result=math.max(result,bfs(i,j))
        end
    end
end
print(result)