from collections import deque

W,H=map(int,input().split())
MAP=[input().strip() for i in range(H)]

Q=deque()

ANS=[[1<<30]*W for i in range(H)]

flag=0

for i in range(H):
    for j in range(W):
        if MAP[i][j]=="." and flag==0:
            Q2=[(i,j)]
            ANS[i][j]=0
            flag=1

            while Q2:
                x,y=Q2.pop()
                Q.append((x,y))
                for z,w in [(x+1,y),(x-1,y),(x,y+1),(x,y-1)]:
                    if 0<=z<H and 0<=w<W and ANS[z][w]==1<<30 and MAP[z][w]==".":
                        ANS[z][w]=0
                        Q2.append((z,w))


while Q:
    x,y=Q.popleft()
    for z,w in [(x+1,y),(x-1,y),(x,y+1),(x,y-1)]:
        if 0<=z<H and 0<=w<W and ANS[z][w]>ANS[x][y]+1:
            ANS[z][w]=ANS[x][y]+1
            Q.append((z,w))

LANS=1<<30

for i in range(H):
    for j in range(W):
        if ANS[i][j]!=0 and MAP[i][j]==".":
            LANS=min(LANS,ANS[i][j])

print(LANS-1)