#define _CRT_SECURE_NO_WARNINGS
#include"bits/stdc++.h"
#ifdef _DEBUG
#define DBG(n) n
#else
#define DBG(n)
#endif
#define INF         1e9
#define INFLL       1e18
#define EPS         1e-9
#define REP(i,n)    for(ll i=0,i##_len=(n);i<i##_len;++i)
#define REP1(i,n)   for(ll i=1,i##_len=(n);i<=i##_len;++i)
#define REPR(i,n)   for(ll i=(n)-1;i>=0;--i)
#define REPR1(i,n)  for(ll i=(n);i>0;--i)
#define REPC(i,obj) for(auto i:obj)
#define ALL(obj)    (obj).begin(),(obj).end()
#define SETP(n)     cout<<fixed<<setprecision(n)
using namespace std;
using ll = long long;
template<typename T = ll>inline T in() { T ret; cin >> ret; return ret; }

signed main()
{
	for (ll n = in(); n != 0; n = in()) {
		vector<vector<ll>>masq(n, vector<ll>(n));
		ll ans = 0;
		REP(i, n) {
			REP(j, n) {
				masq[i][j] = in<char>() == '*' ? 0 : 1;
				if (i * j) ans = max(ans, masq[i][j] = masq[i][j] == 0 ? 0 : min({ masq[i - 1][j - 1],masq[i - 1][j],masq[i][j - 1] }) + 1);
			}
		}
		cout << ans << endl;
	}
}
