import sys
input = sys.stdin.readline
class cs_2d():
    
    def __init__(self,x):
        
            n = len(x)
            m = len(x[0])
            tmp = [[0]*(m+1) for _ in range(n+1)]
            for i in range(n):
                tmp[i+1][1:] = x[i]
                for j in range(m):
                    tmp[i+1][j+1] += tmp[i+1][j]
            
            for j in range(m):
                for i in range(n):
                    tmp[i+1][j+1] += tmp[i][j+1]
                    
            self.S = tmp
    
    def query(self,ix,jx,iy,jy):
        ##[ix,jx)×[iy,jy)
        
        T = self.S
        
        return T[jx][jy] - T[jx][iy] - T[ix][jy] + T[ix][iy]
        
        
H,W = map(int,input().split())
S = [list(input())[:-1] for _ in range(H)]
for i in range(H):
    for j in range(W):
        S[i][j] = int(S[i][j]=='.')
  
  
  
C = cs_2d(S)

def is_ok(x,y,n):
    return C.query(x-n+1,x+n,y-n+1,y+n)==0
    
ans = 1
for i in range(1,H-1):
    for j in range(ans,W-ans):
        if S[i][j] == 1:
            continue
        L = min(i+1,j+1,H-i,W-j)
        if is_ok(i,j,L):
            ans = max(ans,L)
            continue
        if ans>L:
            continue
        y = L
        x = 1
        while y-x>1:
            mid = (y+x)//2
            if is_ok(i,j,mid):
                x = mid
            else:
                y = mid
        ans = max(ans,x)

print(ans)