#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES

#include "bits/stdc++.h"
#define REP(i,a,b) for(i=a;i<b;++i)
#define rep(i,n) REP(i,0,n)
#define ll long long
#define ull unsigned ll
typedef long double ld;
#define ALL(a) (a).begin(),(a).end()
#define ifnot(a) if(not a)
#define dump(x)  cerr << #x << " = " << (x) << endl
using namespace std;

void reader(int &a) { scanf("%d", &a); }
void reader(double &a) { scanf("%lf", &a); }
void reader(char a[]) { scanf("%s", a); }
void reader(char &a) { scanf(" %c", &a); }
void reader(ll &a) { scanf("%lld", &a); }
void reader(ull &a) { scanf("%llu", &a); }
// void reader(string& a){cin >> a;};
template<class T, class U> void reader(T& t, U& u) { reader(t); reader(u); }
template<class T, class U, class V> void reader(T& t, U& u, V& v) { reader(t); reader(u); reader(v); }

void writer(const int a, char c) { printf("%d", a); putchar(c); }
void writer(const ll a, char c) { printf("%lld", a); putchar(c); }
void writer(const ull a, char c) { printf("%llu", a); putchar(c); }
void writer(const double a, char c) { printf("%.20lf", a); putchar(c); }
void writer(const char a[]) { printf("%s", a); };
void writer(const char a[], char c) { printf("%s", a); putchar(c); };
void writer(const char a, char c) { putchar(a); putchar(c); };
template<class T> void writerLn(T t) { writer(t, '\n'); }
template<class T, class U> void writerLn(T t, U u) { writer(t, ' '); writer(u, '\n'); }
template<class T, class U, class V> void writerLn(T t, U u, V v) { writer(t, ' '); writer(u, ' '); writer(v, '\n'); }
template<class T> void writerArr(T x[], int n) { int i; if (!n) { putchar('\n'); return; }rep(i, n - 1) writer(x[i], ' '); writer(x[n - 1], '\n'); }
template<class T> void writerVec(vector<T> x) { int n = x.size(); int i; if (!n) { putchar('\n'); return; }rep(i, n - 1) writer(x[i], ' '); writer(x[n - 1], '\n'); }

vector<string> split(const string &str, char sep) { vector<string> v; stringstream ss(str); string buffer; while (getline(ss, buffer, sep)) { v.push_back(buffer); }return v; }
// #define int ll
bool test = 0;
int dx[] = { 0,1,0,-1 };
int dy[] = { 1,0,-1,0 };
#define INF (1 << 28)
ull mod = (int)1e9 + 7;
//.....................
#define MAX 22



struct Node {
	int y, x;
};
int w, h;
char c[MAX][MAX];
int dm[MAX][MAX];
vector<Node> nodes;
int start;
bool create_distace_matrix() {
	int i, j, k;
	int y, x;
	int cf[MAX][MAX];
	rep(i, nodes.size()) {
		rep(y, h) rep(x, w) {
			if (c[y][x] == 'x') cf[y][x] = -2;
			else cf[y][x] = -1;
		}

		cf[nodes[i].y][nodes[i].x] = 0;
		bool changed = true;
		int current = 0;
		for (; changed; current += 1) {
			changed = false;
			rep(y, h) rep(x, w) {
				if (cf[y][x] == current) {
					rep(j, 4) {
						int ny = y + dy[j];
						int nx = x + dx[j];
						if (ny < 0 || h <= ny) continue;
						if (nx < 0 || w <= nx) continue;
						if (cf[ny][nx] == -1) cf[ny][nx] = current + 1;
						changed = true;
					}
				}
			}
		}
		rep(j, nodes.size()) {
			int distance = cf[nodes[j].y][nodes[j].x];
			if (distance < 0) return false;
			dm[i][j] = distance;
		}
	}
	return true;
}
bool done[MAX];
int ans;
void rec(int n, int idx, int sum) {
	int i;
	if (sum >= ans) return;
	if (n == nodes.size()) {
		if (sum < ans) ans = sum;
		return;
	}
	rep(i, nodes.size()) {
		if (done[i]) continue;
		done[i] = true;
		rec(n + 1, i, sum + dm[idx][i]);
		done[i] = false;
	}
}
signed main(void)
{
	int i, j, k, l;
	while (1) {
		cin >> w >> h;
		if (w == 0) return 0;
		rep(i, h) reader(c[i]);
		nodes.clear();
		rep(i, h) rep(j, w) {
			if (c[i][j] == 'o') start = nodes.size();
			if (c[i][j] == 'o' || c[i][j] == '*') {
				nodes.push_back({ i,j });
			}
		}
		if (!create_distace_matrix()) {
			cout << -1 << endl;
			continue;
		}
		ans = INT_MAX;
		done[start] = true;
		rec(1, start, 0);
		cout << ans << endl;
		done[start] = false;
	}
	return 0;
}

Measured memory_bytes: 3196