def main():
    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(map(lambda x : x == '.',input()))[:-1] for _ in range(H)]
    
    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 C.query(i-mid+1,i+mid,j-mid+1,j+mid)==0:
                    x = mid
                else:
                    y = mid
            ans = max(ans,x)
    
    print(ans)
main()