from itertools import permutations import sys sys.setrecursionlimit(10 ** 6) from bisect import * from collections import * from heapq import * def II(): return int(sys.stdin.readline()) def MI(): return map(int, sys.stdin.readline().split()) def LI(): return list(map(int, sys.stdin.readline().split())) def SI(): return sys.stdin.readline()[:-1] def LLI(rows_number): return [LI() for _ in range(rows_number)] int1 = lambda x: int(x) - 1 def MI1(): return map(int1, sys.stdin.readline().split()) def LI1(): return list(map(int1, sys.stdin.readline().split())) p2D = lambda x: print(*x, sep="\n") dij = [(1, 0), (0, 1), (-1, 0), (0, -1)] def main(): h,w=MI() ss=[SI() for _ in range(h)] # 陸地に出来る最大正方形を見つける # 参考https://qiita.com/gushwell/items/4ab87b52f6511f11f58d # そのマスを右下とする正方形がとれる、最大の辺の長さ qq=[[0]*w for _ in range(h)] for i in range(h): for j in range(w): if ss[i][j]=="#": if i*j:qq[i][j]=min(qq[i-1][j-1],qq[i-1][j],qq[i][j-1])+1 else:qq[i][j]=1 #p2D(qq) mx=max(max(row) for row in qq) print((mx+1)//2) main()