#include<cstdio>
#include<iostream>
#include<vector>
#include<set>
#include<map>
#include<string>
#include<stack>
#include<queue>
#include<cstdlib>
#include<cmath>
#include<utility>
#include<algorithm>
using namespace std;
#define lli long long int
#define endl "\n"
int mv[]={0,0,1,-1};
int mh[]={1,-1,0,0};
int graph[21][21];
int ds[11][11];
int caminhos[10];
void bfs(int suj1, map<int,int> &sujeira, int n, int m,  int inicio)
{
  vector<bool> usados(n*m,false);
  int  mv[]={0,0,1,-1};
  int  mh[]={1,-1,0,0};
  queue<int> filona;
  filona.push(suj1);
  vector<int> distancia(n*m,0);
  while(!filona.empty())
  {
    int at=filona.front();
    usados[at]=true;
    filona.pop();
    int x=at%n;
    int y=at/n;
    for(int i=0;i<4;i++)
    {
      int atx=x+mh[i];
      int aty=y+mv[i];
      if(atx >= 0 and atx < n and aty >= 0 and aty < m)
      {
        int f= atx + aty*n;
        if(graph[aty][atx]==1) continue;
        if(!usados[f] and graph[aty][atx]==0)
        {
          filona.push(f);
          distancia[f]=distancia[at]+1;
          if(sujeira.count(f)!=0)
          {
            ds[sujeira[suj1]][sujeira[f]]=distancia[f];
            ds[sujeira[f]][sujeira[suj1]]=distancia[f];
          }
          if(f==inicio)
          {
            ds[sujeira[suj1]][0]=ds[0][sujeira[suj1]]=distancia[f];
          }
        }
      }
    }
  }
}
int fb(int n)
{
  int menor=10000;
  do {
    int ini=0;
    int aux=0;
    int prox=0;
    for(int i=0;i<n;i++)
    {
      prox=ds[ini][caminhos[i]];
      aux+=prox;
      ini=caminhos[i];
    }
  menor=min(menor,aux);
  }while(next_permutation(caminhos,caminhos+n));
  return menor;
}
int main()
{
  ios::sync_with_stdio(false);
  cin.tie(0); cout.tie(0);
  int n,m;
  while(1)
  {
    cin>>n>>m;
    if(n==0 and m==0) break;
    int inicio=0;
    map<int,int> sujeira;
    int k=1;
    for(int i=0;i<m;i++)
    {
      for(int j=0;j<n;j++)
      {
        char p;
        cin>>p;
        if(p=='o')
        {
          graph[i][j]=0;
          inicio= i*n+j;
        }
        if(p=='*')
        {
          int a=0;
          graph[i][j]=0;
          a= i*n + j;
          sujeira.insert(make_pair(a,k));
          k++;
        }
        if(p=='x')
        {
          graph[i][j]=1;
        }
        if(p=='.')
          graph[i][j]=0;
      }
    }
    bool taerrado=false;
    if(sujeira.size()==0)
    {
      cout<<"0"<<endl;
      continue;
    }
    map<int,int>::iterator it;
    for(int i=0;i<=sujeira.size();i++)
      for(int j=0;j<=sujeira.size();j++) ds[i][j]=0;
    for(it= sujeira.begin() ; it!= sujeira.end() ; ++it)
    {
      bfs(it->first,  sujeira, n,m,inicio);
    }
/*    for(int i=0;i<=sujeira.size();i++)
    {
      for(int j=0;j<=sujeira.size();j++)
      {
        cout<<ds[i][j]<<" ";
      }
      cout<<endl;
    }*/

    int v=0;
    for(int i=0;i<=sujeira.size();i++)
    {
      v=0;
      for(int j=0;j<=sujeira.size();j++)
      {
        if(ds[i][j]==0)
          v++;
      }
      if(v>1)
      {
        taerrado=true;
        break;
      }
    }
    if(!taerrado)
    {
      for(int i=0;i<sujeira.size();i++) caminhos[i]=i+1;
      cout<<fb(sujeira.size())<<endl;
    }
    else cout<<"-1"<<endl;
  }

  return 0;
}