h,w=map(int,input().split())
l=[]
for i in range(h):
    l.append(list(input()))
for i in range(h):
    for j in range(w):
        if l[i][j]=='.':
            s=[i,j]
v=[[0 for i in range(w)] for j in range(h)]       
v[s[0]][s[1]]=1
mc=0
ms=s
q=[[s,0]]
while len(q):
    p=q.pop(0)
    x=p[0][0]
    y=p[0][1]
    c=p[1]
    k=[[0,1],[0,-1],[1,0],[-1,0]]
    for j in k:
        nx=x+j[0]
        ny=y+j[1]
        if 0<=nx<h and 0<=ny<w and l[nx][ny]=='.' and v[nx][ny]==0:
            v[nx][ny]=1
            q.append([[nx,ny],c+1])
            if mc<c+1:
                mc=c+1
                ms=[nx,ny]
v=[[0 for i in range(w)] for j in range(h)]       
v[ms[0]][ms[1]]=1
ans=0
q=[[ms,0]]
while len(q):
    p=q.pop(0)
    x=p[0][0]
    y=p[0][1]
    c=p[1]
    k=[[0,1],[0,-1],[1,0],[-1,0]]
    for j in k:
        nx=x+j[0]
        ny=y+j[1]
        if 0<=nx<h and 0<=ny<w and l[nx][ny]=='.' and v[nx][ny]==0:
            v[nx][ny]=1
            q.append([[nx,ny],c+1])
            ans=max(ans,c+1)
                #ms=[nx,ny]
print(ans)
