#include <string>
#include <vector>
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<stack>
#include<queue>
#include<cmath>
#include<algorithm>
#include<functional>
#include<list>
#include<deque>
#include<bitset>
#include<set>
#include<map>
#include<unordered_map>
#include<cstring>
#include<sstream>
#include<complex>
#include<iomanip>
#include<numeric>
#include<cassert>
#define X first
#define Y second
#define pb push_back
#define rep(X,Y) for (int (X) = 0;(X) < (Y);++(X))
#define reps(X,S,Y) for (int (X) = S;(X) < (Y);++(X))
#define rrep(X,Y) for (int (X) = (Y)-1;(X) >=0;--(X))
#define repe(X,Y) for ((X) = 0;(X) < (Y);++(X))
#define peat(X,Y) for (;(X) < (Y);++(X))
#define all(X) (X).begin(),(X).end()
#define rall(X) (X).rbegin(),(X).rend()
#define eb emplace_back
#define UNIQUE(X) (X).erase(unique(all(X)),(X).end())

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
template<class T> using vv=vector<vector<T>>;
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
os<<"{"; rep(i,t.size()) {os<<t[i]<<",";} os<<"}"<<endl; return os;}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}
template<class T> inline bool MX(T &l,const T &r){return l<r?l=r,1:0;}
template<class T> inline bool MN(T &l,const T &r){return l>r?l=r,1:0;}
const ll MOD=1e9+7;

typedef ll Cost;
struct Edge{int to,cap,rev;Cost cost;};
typedef vector<Edge> Edges;
typedef vector<vector<Edge> > Graph;

void add_edge(Graph &g,int from,int to,int cap,Cost cost=0){
  g[from].pb((Edge){to,cap,(int)g[to].size(),cost});
  g[to].pb((Edge){from,0,(int)g[from].size()-1,-cost});
}
Cost INF=2e7;

//max_flow

vector<int> used;

int dfs(Graph &g,int v,int t,int f){
  if(v==t) return f;
  used[v]=1;
  int re,i,j,k;
  for(i=0;i<g[v].size();i++){
    Edge &e=g[v][i];
    if(e.cap<=0||used[e.to])continue;
    int d=dfs(g,e.to,t,min(f,e.cap));
    if(d>0){
      e.cap-=d;
      g[e.to][e.rev].cap+=d;
      return d;
    }
  }
  return 0;
}

int max_flow(Graph &g,int s,int t){
  int sum=0,f,V=g.size();
  while(1){
    used.resize(V);
    fill(all(used),0);
    sum+=f=dfs(g,s,t,INF);
    if(!f)return sum;
  }
}
int w,h;
int fun(int x,int y,int t=0){
  return x+y*w+t*w*h;
}


int main(){
  ios_base::sync_with_stdio(false);
  cout<<fixed<<setprecision(0);
  cin>>h>>w;
  Graph g(h*w*2+2);
  rep(i,h)rep(j,w){
    if(i+1<h){
      add_edge(g,fun(j,i,1),fun(j,i+1,0),INF);
      add_edge(g,fun(j,i+1,1),fun(j,i,0),INF);
    }
    if(j+1<w){
      add_edge(g,fun(j,i,1),fun(j+1,i,0),INF);
      add_edge(g,fun(j+1,i,1),fun(j,i,0),INF);
    }
  }

  vector<string> mp(h);
  rep(i,h) cin>>mp[i];
  
  int f=0;
  rep(i,h){
    add_edge(g,fun(0,i,1),h*w*2+1,INF);
    add_edge(g,fun(w-1,i,1),h*w*2+1,INF);
    if(mp[i][0]=='X' || mp[i][w-1]=='X') f=1;
  }
  rep(i,w){
    add_edge(g,fun(i,0,1),h*w*2+1,INF);
    add_edge(g,fun(i,h-1,1),h*w*2+1,INF);
    if(mp[0][i]=='X' || mp[h-1][i]=='X') f=1;
  }
  if(h==1 || w==1 || f){
    cout<<-1<<endl;
    return 0;
  }

  rep(i,h)rep(j,w){
    if(mp[i][j]=='X'){
      add_edge(g,h*w*2,fun(j,i,0),INF);
      add_edge(g,fun(j,i,0),fun(j,i,1),INF);
    }else{
      add_edge(g,fun(j,i,0),fun(j,i,1),1);
    }
  }
  int re=max_flow(g,h*w*2,h*w*2+1);
  cout<<min(re,(h+w-4)*2)<<endl;
  return 0;
}


Measured memory_bytes: 6144