#include <iostream>
#include <cstring>
#include <map>
#include <vector>

using namespace std;

struct Point {
  Point() {}
  Point(int xx, int yy) : x(xx), y(yy) {}
  Point(const Point& p) : x(p.x), y(p.y) {}
  int x, y;
};

int w, h;
char field[32][32];
int table[32][32];
Point start;
vector<Point> dirty;
int dn;
int dist[10][10];
int dstart[10];

void MakeTable(Point s) {
  memset(table, -1, sizeof(table));
  multimap<int, Point> up;
  up.insert(make_pair(0, s));
  while(!up.empty()) {
    int d = up.begin()->first;
    Point p = up.begin()->second;
    int i = p.x; int j = p.y;
    up.erase(up.begin());
    if(i < 0 || h <= i || j < 0 || w <= j)
      continue;
    if(field[i][j] == 'x' || table[i][j] >= 0)
      continue;
    //cout << d << i << j << endl;
    table[i][j] = d;
    up.insert(make_pair(d+1, Point(i+1,j)));
    up.insert(make_pair(d+1, Point(i-1,j)));
    up.insert(make_pair(d+1, Point(i,j+1)));
    up.insert(make_pair(d+1, Point(i,j-1)));
  }
}

int Solve(int state, int from, int to) {
  if(state == 0) {
    return dstart[to];
  }
  int next = state & ~(1<<from);
  if(next == 0) {
    int t = Solve(next, 0, from) + dist[from][to];
    //cout << state << " " << from << " " << to << " " << t << endl;
    return t;
  }
  int ans = -1;
  for(int i = 0; i < dn; ++i) {
    if(next & (1<<i)) {
      int t = Solve(next, i, from) + dist[from][to];
      //cout << state << " " << from << " " << to << " " << t << endl;
      if(ans < 0 || t < ans) {
	ans = t;
      }
    }
  }
  return ans;
}

int main() {
  while(cin >> w >> h >> ws && w) {
    dirty.clear();
    for(int i = 0; i < h; ++i) {
      cin.getline(field[i], 32);
      for(int j = 0; j < w; ++j) {
	switch(field[i][j]) {
	case 'o':
	  start.x = i;
	  start.y = j;
	  break;
	case '*':
	  dirty.push_back(Point(i,j));
	  break;
	};
      }
    }
    bool unsolve = false;
    dn = dirty.size();
    MakeTable(start);
    for(int i = 0; i < dn; ++i) {
      Point p = dirty[i];
      dstart[i] = table[p.x][p.y];
      if(dstart[i] < 0)
	unsolve = true;
    }
    if(unsolve) {
      cout << -1 << endl;
      continue;
    }
    for(int i = 0; i < dn; ++i) {
      MakeTable(dirty[i]);
      for(int j = 0; j < dn; ++j) {
	Point p = dirty[j];
	dist[i][j] = table[p.x][p.y];
	//cout << dist[i][j] << " ";
      }
      //cout << endl;
    }

    int ans = -1;
    for(int i = 0; i < dn; ++i) {
      int t = Solve((1<<dn)-1, i, i);
      if(ans < 0 || t < ans)
	ans = t;
    }
    cout << ans << endl;
  }
}