H, W = map(int, input().split())
curr = [0] * (W + 2)
maxLen = 0
for r in range(H):
    s = input()
    prev, curr = curr, [0] * (W + 2)
    for c in range(1, W + 1):
        if s[c - 1] == "#":
            curr[c] = min(prev[c - 1], prev[c], curr[c - 1]) + 1
            maxLen = max(maxLen, curr[c])
print((maxLen + 1) // 2)