#include <bits/stdc++.h>
#define FOR(v, a, b) for(int v = (a); v < (b); ++v)
#define FORE(v, a, b) for(int v = (a); v <= (b); ++v)
#define REP(v, n) FOR(v, 0, n)
#define REPE(v, n) FORE(v, 0, n)
#define REV(v, a, b) for(int v = (a); v >= (b); --v)
#define ALL(x) (x).begin(), (x).end()
#define ITR(it, c) for(auto it = (c).begin(); it != (c).end(); ++it)
#define RITR(it, c) for(auto it = (c).rbegin(); it != (c).rend(); ++it)
#define EXIST(c,x) ((c).find(x) != (c).end())
#define LLI long long int
#define fst first
#define snd second

#ifdef DEBUG
#include <misc/C++/Debug.cpp>
#else
#define dump(x) ((void)0)
#endif

#define gcd __gcd

using namespace std;
template <class T> constexpr T lcm(T m, T n){return m/gcd(m,n)*n;}

template <typename I> void join(ostream &ost, I s, I t, string d=" "){for(auto i=s; i!=t; ++i){if(i!=s)ost<<d; ost<<*i;}ost<<endl;}
template <typename T> istream& operator>>(istream &is, vector<T> &v){for(auto &a : v) is >> a; return is;}
template <typename T, typename U> istream& operator>>(istream &is, pair<T,U> &p){is >> p.first >> p.second; return is;}

template <typename T, typename U> T& chmin(T &a, const U &b){return a = (a<=b?a:b);}
template <typename T, typename U> T& chmax(T &a, const U &b){return a = (a>=b?a:b);}
template <typename T, size_t N, typename U> void fill_array(T (&a)[N], const U &v){fill((U*)a, (U*)(a+N), v);}

template <typename T, T INF> class FordFulkerson{ 
  struct edge{
    int to, rev;
    T cap;
  };

  int size;

  vector<vector<edge>> graph;
  vector<bool> visit;
  
  T dfs(int from, int to, T flow){
    if(from == to) return flow;
    visit[from] = true;

    for(auto &e : graph[from]){
      if(!visit[e.to] and e.cap > 0){
        T d = dfs(e.to, to, min(flow, e.cap));
	if(d > 0){
	  e.cap -= d;
	  graph[e.to][e.rev].cap += d;
	  return d;
	}
      }
    }
    return 0;
  }
  
public:
  FordFulkerson(int size): size(size), graph(size), visit(size){}

  void add_edge(int from, int to, const T &cap){
    graph[from].push_back((edge){to, (int)graph[to].size(), cap});
    graph[to].push_back((edge){from, (int)graph[from].size()-1, 0});
  }

  T max_flow(int s, int t){
    T ret = 0;

    while(1){
      visit.assign(size,false);
      T flow = dfs(s,t,INF);
      if(flow == 0) return ret;
      ret += flow;
    }
  }
};



const int dir4[4][2] = {{1,0},{-1,0},{0,1},{0,-1}};
const LLI inf = LLONG_MAX / 10000;

int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  
  int H,W;
  while(cin >> H >> W){
    vector<string> S(H); cin >> S;

    FordFulkerson<LLI,inf> mf(H*W*2+2);
    int s = H*W*2, t = s+1;
    vector<vector<int>> in(H,vector<int>(W)), out(H,vector<int>(W));
    REP(i,H) REP(j,W){
      in[i][j] = i+j*H;
      out[i][j] = in[i][j] + H*W;
    }
    
    REP(i,H){
      REP(j,W){
	if(i==0 or i==H-1 or j==0 or j==W-1) mf.add_edge(out[i][j], t, inf);

	if(S[i][j] == 'X'){
	  mf.add_edge(s, in[i][j], inf);
	  mf.add_edge(in[i][j], out[i][j], inf);
	}
	else mf.add_edge(in[i][j], out[i][j], 1);

	for(auto &d : dir4){
	  int y = i+d[0], x = j+d[1];
	  if(x<0 or x>=W or y<0 or y>=H) continue;
	  mf.add_edge(out[i][j], in[y][x], inf);
	}
      }
    }

    LLI ans = mf.max_flow(s,t);
    
    cout << (ans>=inf ? -1 : ans) << endl;
  }
  
  return 0;
}


Measured memory_bytes: 3836