### BUGGY CODE ###

# -*- coding: utf-8 -*-
"""
http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0092
"""
import sys



def find_square(data):
    max_size = 0
    lmap = []
    for row in data:
        temp = [0]
        for c in row:
            if c == '.':
                temp.append(1)
            else:
                temp.append(0)
        lmap.append(temp)
        temp = [0]

    for y in range(1, len(lmap)):
        for x in range(1, len(lmap[0])):
            if lmap[y][x] == 1:
                if lmap[y-1][x-1] != 0 and lmap[y-1][x] != 0 and lmap[y][x-1] != 0:
                    lmap[y][x] = min(lmap[y-1][x-1], min(lmap[y-1][x], lmap[y][x-1])) + 1
                    if lmap[y][x] > max_size:
                        max_size = lmap[y][x]
    return max_size


def main(args):
    while True:
        n = int(input())
        if n == 0:
            break
        data = [input() for _ in range(n)]
        result = find_square(data)
        print(result)


if __name__ == '__main__':
    main(sys.argv[1:])

### FIXED CODE ###

# -*- coding: utf-8 -*-
"""
http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0092
"""
import sys


def find_square0(data):
    max_size = 0
    dp = [] # dp??¨???2?¬??????????
    # '.'????????????1??????'*'????????????0????????????
    for row in data:
        temp = []
        for c in row:
            if c == '.':
                temp.append(1)
            else:
                temp.append(0)
        dp.append(temp)

    # ????±?????????????????????????????????£????????????????????£?????¢????¢??????§??????????????§????????????
    for y in range(1, len(dp)):
        for x in range(1, len(dp[0])):
            if dp[y][x] == 1:
                dp[y][x] = min(dp[y-1][x-1], dp[y-1][x], dp[y][x-1]) + 1
                if dp[y][x] > max_size:
                    max_size = dp[y][x]
    return max_size


def find_square2(data):
    max_size = 0
    dp = [[0]*len(data[0]) for _ in range(len(data))] # dp??¨???2?¬???????????????¨??????0??§?????????
    # '.'????????????1???
    for y, row in enumerate(data):
        for x, c in enumerate(row):
            if c == '.':
                dp[y][x] = 1

    # (?????¨???(curr_row)??¨???????????????(prev_row)????????¢????????????????????§?????????????????????)
    prev_row = dp[0]
    for curr_row in dp[1:]:
        for x, t in enumerate(curr_row[1:], start=1):
            if t == 1:
                curr_row[x] = min(prev_row[x-1], prev_row[x], curr_row[x-1]) + 1
                if curr_row[x] > max_size:
                    max_size = curr_row[x]
        prev_row = curr_row
    return max_size


def find_square3(data):
    from array import array
    max_size = 0
    dp = [array('I', [0]*len(data[0])) for _ in range(len(data))]  # ????¬?????????????array??????????????§?¢????
    # '.'????????????1???
    for y, row in enumerate(data):
        for x, c in enumerate(row):
            if c == '.':
                dp[y][x] = 1

    prev_row = dp[0]
    for curr_row in dp[1:]:
        for x, t in enumerate(curr_row[1:], start=1):
            if t == 1:
                curr_row[x] = min(prev_row[x-1], prev_row[x], curr_row[x-1]) + 1
                if curr_row[x] > max_size:
                    max_size = curr_row[x]
        prev_row = curr_row
    return max_size


def find_square4(data):
    # 4.91[s]
    max_size = 0
    dp = [0 for _ in range(1024*1024)]
    # '.'????????????1???
    for y, row in enumerate(data):
        for x, c in enumerate(row):
            if c == '.':
                dp[y*1024+x] = 1

    # ????±?????????????????????????????????£????????????????????£?????¢????¢??????§??????????????§????????????
    for y in range(1, len(data)):
        for x in range(1, len(data)):
            if dp[y*1024+x] == 1:
                dp[y*1024+x] = min(dp[(y-1)*1024+x-1], dp[(y-1)*1024+x], dp[y*1024+x-1]) + 1
                if dp[y*1024+x] > max_size:
                    max_size = dp[y*1024+x]
    return max_size



def main(args):
    while True:
        n = int(input())
        if n == 0:
            break
        data = [input() for _ in range(n)]
        result = find_square2(data)
        print(result)


if __name__ == '__main__':
    main(sys.argv[1:])