#include <algorithm>
#include <iostream>
#include <sstream>
#include <cstdlib>
#include <climits>
#include <cstring>
#include <iomanip>
#include <limits>
#include <bitset>
#include <locale>
#include <cstdio>
#include <vector>
#include <cfloat>
#include <cmath>
#include <stack>
#include <queue>
#include <deque>
#include <ctime>
#include <list>
#include <map>
#include <set>

using namespace std;

#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define sz(x) (int) x.size ()

typedef pair <int, int> ii;
typedef pair <int, ii> iii;
typedef vector <int> vi;
typedef vector <vi> vvi;
typedef vector <ii> vii;
typedef vector <vii> vvii;

int m, n;
char mat [22][22];
int d [22][22], c [11][11];
int dx [] = {-1 ,0, 0, 1};
int dy [] = {0, -1, 1, 0};

inline void bfs (const int &x, const int &y)
{
	memset (d, -1, sizeof d);
	d [x][y] = 0;
	queue <ii> q;
	q.push (mp (x, y));
	while (sz (q))
	{
		ii u = q.front (); q.pop ();
		for (int i = 0; i < 4; ++i)
		{
			ii v (u.fi + dx [i], u.se + dy [i]);
			if (0 <= v.fi && v.fi < m && 0 <= v.se && v.se < n && mat [v.fi][v.se] != 'x' && d [v.fi][v.se] == -1)
			{
				d [v.fi][v.se] = d [u.fi][u.se] + 1;
				q.push (v);
			}
		}
	}
}

int dp [1 << 11][11];

int main ()
{
	while (scanf ("%d%d", &n, &m), m)
	{
		vii dust;
		int s;
		for (int i = 0; i < m; ++i)
		{
			scanf ("%s", mat [i]);
			for (int j = 0; j < n; ++j)
				if (mat [i][j] == '*' || mat [i][j] == 'o')
				{
					if (mat [i][j] == 'o') s = sz (dust);
					dust.pb (mp (i, j));
				}
		}
		int k = sz (dust);
		bool ok = true;
		for (int i = 0; i < k; ++i)
			for (int j = 0; j < i; ++j)
			{
				bfs (dust [i].fi, dust [i].se);
				if ((c [i][j] = c [j][i] = d [dust [j].fi][dust [j].se]) == -1) ok = false;
			}
		if (ok)
		{
			/*
			for (int i = 0; i < k; ++i)
			{
				for (int j = 0; j < k; ++j)
					printf ("%d ", c [i][j]);
				putchar ('\n');
			}
			*/
			int f = (1 << k) - 1;
			for (int i = 1; i <= f; ++i)
				for (int j = 0; j < k; ++j)
					dp [i][j] = 1 << 30;
			dp [1 << s][s] = 0;
			for (int S = 1; S <= f; ++S)
				for (int i = 0; i < k; ++i)
					if (S & (1 << i))
						for (int j = 0; j < k; ++j)
							if (i != j && S & (1 << j))
								dp [S][i] = min (dp [S][i], dp [S - (1 << i)][j] + c [i][j]);
			int res = INT_MAX;
			for (int i = 0; i < k; ++i)
				if (i != s)
					res = min (res, dp [f][i]);
			printf ("%d\n", res);
		}
		else printf ("-1\n");
	}
	return 0;
}