#include <bits/stdc++.h>
using namespace std;
using LL = long long;

#define fs first
#define sc second

const LL MOD = 1e9+7;
const double EPS = 1e-10;

template<class T>bool chmax(T &a, const T &b) {
    if (a < b) { a = b; return true; }
    return false;
}
template<class T>bool chmin(T &a, const T &b) {
    if (b < a) { a = b; return true; }
    return false;
}

// std::vector Declaration
template<typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts) {
    return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

// std::vector Declaration and Initialization
template<typename T>
vector<T> make_vector(size_t a, T x) { return vector<T>(a, x); }
template<typename T, typename U, typename... Ts>
auto make_vector(size_t a, U b, Ts... ts) {
    return vector<decltype(make_vector<T>(b,ts...))>(a,make_vector<T>(b,ts...));
}

// std::vector Input
template<typename T>
istream& operator>>(istream& is, vector<T>& v) {
    for (auto &e : v) {
        is >> e;
    }
    return is;
}

// std::vector Debug
template<typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    os << "[";
    bool a = 1;
    for (auto e : v) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::array Debug
template<typename T, size_t n>
ostream& operator<<(ostream& os, const array<T, n>& v) {
    os << "[";
    bool a = 1;
    for (auto e : v) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::deque Debug
template<typename T>
ostream& operator<<(ostream& os, const deque<T>& d) {
    os << "[";
    bool a = 1;
    for (auto e : d) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::pair Debug
template<typename T, typename U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
    os << "(" << p.first << " " << p.second << ")";
    return os;
}

// std::set Debug
template<typename T>
ostream& operator<<(ostream& os, const set<T>& st) {
    os << "{";
    bool a = 1;
    for (auto e : st) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "}";
    return os;
}

// std::multiset Debug
template<typename T>
ostream& operator<<(ostream& os, const multiset<T>& st) {
    os << "{";
    bool a = 1;
    for (auto e : st) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "}";
    return os;
}

// std::map Debug
template<typename T, typename U>
ostream& operator<<(ostream& os, const map<T, U>& mp) {
    os << "{";
    bool a = 1;
    for (auto e : mp) {
        os << (a ? "" : " ");
        os << e.first << ":" << e.second;
        a = 0;
    }
    os << "}";
    return os;
}

// std::tuple Debug
template<int N, class Tuple>
void out(ostream& os, const Tuple& t){}
template<int N, class Tuple, class H, class ...Ts>
void out(ostream& os, const Tuple& t) {
    if (N) os << " ";
    os << get<N>(t);
    out<N+1,Tuple,Ts...>(os, t);
}
template<class ...Ts>
ostream& operator<<(ostream& os, const tuple<Ts...>& t) {
    os << "(";
    out<0,tuple<Ts...>,Ts...>(os, t);
    os << ")";
    return os;
}

// Debug
#define DUMP(x) cerr<<#x<<" = "<<(x)<<endl

// Weighted edge
template<typename T>
struct edge {
    int src, to;
    T cost;

    edge(int to, T cost) : src(-1), to(to), cost(cost) {}
    edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

    edge &operator=(const int &x) {
        to = x;
        return *this;
    }

    operator int() const { return to; }

    friend ostream& operator<<(ostream& os, const edge& e) {
        return os << "(" << e.src << "->" << e.to << ":" << e.cost << ")";
    }
};

template<typename T>
using Edges = vector<edge<T>>;
template<typename T>
using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template<typename T>
using MatrixGraph = vector<vector<T>>;


template<typename T>
void dijkstra(
        const WeightedGraph<T> &g,
        vector<T> &dist,
        int s)
{
    const auto INF = numeric_limits<T>::max();
    dist.assign(g.size(), INF);

    using Pi = pair< T, int >;
    priority_queue<Pi, vector<Pi>, greater<Pi>> que;
    dist[s] = 0;
    que.emplace(dist[s], s);
    while(!que.empty()) {
        T cost;
        int idx;
        tie(cost, idx) = que.top();
        que.pop();
        if(dist[idx] < cost) continue;
        for(auto &e : g[idx]) {
            auto next_cost = cost + e.cost;
            if(dist[e.to] <= next_cost) continue;
            dist[e.to] = next_cost;
            que.emplace(dist[e.to], e.to);
        }
    }
}

int main()
{
    while (true) {
        int w, h; cin >> w >> h;
        if (w == 0 && h == 0) {
            break;
        }

        vector<pair<int,int>> dirty;
        pair<int,int> start;
        vector<string> c(h);
        for (int i = 0; i < h; ++i) {
            cin >> c[i];
            for (int j = 0; j < w; ++j) {
                if (c[i][j] == '*') {
                    dirty.emplace_back(i, j);
                }
                if (c[i][j] == 'o') {
                    start = {i, j};
                }
            }
        }

        int m = dirty.size();
        WeightedGraph<int> G(h*w*(1<<m));
        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                int k = distance(
                        dirty.begin(),
                        find(dirty.begin(), dirty.end(), make_pair(i, j)));
                for (int s = 0; s < 1<<m; ++s) {
                    if (k < m) {
                        G[i*w*(1<<m) + j*(1<<m) + s].emplace_back(
                                i*w*(1<<m) + j*(1<<m) + (s | (1<<k)), 0);
                    }

                    int dx[4] = {0, 1, 0, -1},
                        dy[4] = {1, 0, -1, 0};
                    for (int u = 0; u < 4; ++u) {
                        int y = i + dy[u],
                            x = j + dx[u];
                        if (y < 0 || h <= y || x < 0 || w <= x) {
                            continue;
                        }
                        if (c[y][x] == 'x') continue;
                        G[i*w*(1<<m) + j*(1<<m) + s].emplace_back(
                                y*w*(1<<m) + x*(1<<m) + s, 1);
                    }
                }
            }
        }

        vector<int> dist;
        int s = start.fs*w*(1<<m) + start.sc*(1<<m);
        dijkstra(G, dist, s);

        int ans = numeric_limits<int>::max();
        for (int i = 0; i < h; ++i) {
            for (int j = 0; j < w; ++j) {
                chmin(ans, dist[i*w*(1<<m) + j*(1<<m) + (1<<m)-1]);
            }
        }

        if (ans == numeric_limits<int>::max()) {
            ans = -1;
        }

        cout << ans << endl;
    }

    return 0;
}



Measured memory_bytes: 18820