# Author: cr4zjh0bp
# Created: Tue Mar 17 19:50:06 UTC 2020
import sys
 
stdin = sys.stdin
inf = 1 << 60
mod = 1000000007
 
ni      = lambda: int(ns())
nin     = lambda y: [ni() for _ in range(y)]
na      = lambda: list(map(int, stdin.readline().split()))
nan     = lambda y: [na() for _ in range(y)]
nf      = lambda: float(ns())
nfn     = lambda y: [nf() for _ in range(y)]
nfa     = lambda: list(map(float, stdin.readline().split()))
nfan    = lambda y: [nfa() for _ in range(y)]
ns      = lambda: stdin.readline().rstrip()
nsn     = lambda y: [ns() for _ in range(y)]
ncl     = lambda y: [list(ns()) for _ in range(y)]
nas     = lambda: stdin.readline().split()

h, w = na()
m = nsn(h)

dist = [[inf] * (h * w) for _ in range(h * w)]
dx, dy = [0, 1, -1, 0], [1, 0, 0, -1]
for i in range(h):
    for j in range(w):
        for k in range(4):
            ni, nj = i + dx[k], j + dy[k]
            if ni >= 0 and ni < h and nj >= 0 and nj < w and m[ni][nj] != '#' and m[i][j] != '#':
                dist[i * h + j][ni * h + nj] = 1
        dist[i * h + j][i * h + j] = 0

def floyd_warshall(n, dist):
    for i in range(n):
        for j in range(n):
            for k in range(n):
                dist[j][k] = min(dist[j][k], dist[j][i] + dist[i][k])

floyd_warshall(h * w, dist)

ans = 0
for i in range(h * w):
    for j in range(h * w):
        if dist[i][j] != inf:
            ans = max(ans, dist[i][j])

print(ans)