#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <cassert>
#include <iostream>
#include <cctype>
#include <sstream>
#include <string>
#include <list>
#include <vector>
#include <queue>
#include <set>
#include <stack>
#include <map>
#include <utility>
#include <numeric>
#include <algorithm>
#include <iterator>
#include <bitset>
#include <complex>
#include <fstream>
#include <iomanip>
using namespace std;
typedef long long ll;
const double EPS = 1e-9;
typedef vector<int> vint;
typedef pair<int, int> pint;
#define rep(i, n) REP(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define MSG(a) cout << #a << " " << a << endl;
#define REP(i, x, n) for(i = x; i < n; i++)
template<class T> T RoundOff(T a){ return int(a+.5-(a<0)); }
template<class T, class C> void chmax(T& a, C b){ if(a < b) a = b; }
template<class T, class C> void chmin(T& a, C b){ if(b < a) a = b; }
template<class T, class C> pair<T, C> mp(T a, C b){ return make_pair(a, b); }

int main()
{
	int n;

	while(cin >> n && n)
	{
		vector<string> field(n);
		vector<vint>   table(n, vint(n));
		int x, y, i, lim;

		rep(y, n)
			cin >> field[y];

		rep(y, n) rep(x, n)
		{
			if(y == 0 && x == 0)
				table[y][x] = field[y][x] == '*';
			else if(y == 0)
				table[y][x] = table[y][x-1] + (field[y][x] == '*');
			else if(x == 0)
				table[y][x] = table[y-1][x] + (field[y][x] == '*');
			else
				table[y][x] = table[y][x-1] + table[y-1][x] - table[y-1][x-1] + (field[y][x] == '*');
		}

		int res = 0, n2 = n / 2;
		for(y = 0; y < n; y++) for(x = 0; x < n; x++)
		{
			lim = min(n-y, n-x);
			rep(i, lim)
			{
				if(table[y + i][x + i] - table[y][x + i] - table[y + i][x] + table[x][y] == 0)
				{
					chmax(res, i);
				}
			}
		}

		cout << res << endl;
	}
}