//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#define rep(i, n) for(int i=0; i<n; ++i)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
using namespace std;
using ll = int64_t;
using ld = long double;
using P = pair<int, int>;
using vs = vector<string>;
using vi = vector<int>;
using vvi = vector<vi>;
template<class T> using PQ = priority_queue<T>;
template<class T> using PQG = priority_queue<T, vector<T>, greater<T> >;
const int INF = 100010001;
const ll LINF = (ll)INF*INF/10000;
template<typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {return a < b && (a = b, true);}
template<typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {return a > b && (a = b, true);}
template<typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) { return is >> p.first >> p.second;}
template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) { return os << p.first << ' ' << p.second;}

struct edge{
  int to, rev;
  int64_t cap;
  edge(int t=0, int64_t c=0, int r=0):to(t), cap(c), rev(r) {}
};

struct max_flow {
  vector<vector<edge> > G;
  vector<int> level, iter;
  int n;
  max_flow(int n=0):n(n), G(n), level(n), iter(n) {}
  inline void add(int from, int to, int64_t cap) {
    G[from].emplace_back(to, cap, G[to].size());
    G[to].emplace_back(from, 0, G[from].size()-1);
  }
  void bfs(int s) {
    for(int i = 0; i < n; i++) level[i] = -1;
    queue<int> que;
    level[s] = 0;
    que.emplace(s);
    while(!que.empty()) {
      int now = que.front();
      que.pop();
      for(edge &e:G[now]) {
        if(e.cap > 0 and level[e.to] < 0) {
          level[e.to] = level[now] + 1;
          que.emplace(e.to);
        }
      }
    }
  }
  int64_t dfs(int v, int t, int64_t f) {
    if(v == t) return f;
    for(int &i = iter[v]; i < G[v].size(); i++) {
      edge &e = G[v][i];
      if(e.cap > 0 and level[v] < level[e.to]) {
        int64_t d = dfs(e.to, t, min(f, e.cap));
        if(d > 0) {
          e.cap -= d;
          G[e.to][e.rev].cap += d;
          return d;
        }
      }
    }
    return 0;
  }
  int64_t Dinic(int s, int t) {
    int64_t flow = 0;
    while(true) {
      bfs(s);
      if(level[t] < 0) return flow;
      for(int i = 0; i < n; i++) iter[i] = 0;
      int64_t f;
      while((f = dfs(s, t, INT64_MAX)) > 0) flow += f;
    }
  }
};

//head

int h, w;
string ss[100];
int di[] = {0, 1, 0, -1};
int dj[] = {1, 0, -1, 0};

inline bool able(int i, int j) {
  return i >= 0 and j >= 0 and i < h and j < w;
}

inline int num(int i, int j) {
  return j*h+i;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> h >> w;
  rep(i, h) cin >> ss[i];

  max_flow mf(h*w*2+2);
  const int s = h*w*2, t = s+1;
  rep(i, h*w) mf.add(i*2, i*2+1, 1);
  rep(i, h) rep(j, w) {
    if(i*j == 0 or i == h-1 or j == w-1) {
      mf.add(num(i, j)*2+1, t, LINF);
    }
    if(ss[i][j] == 'X') {
      mf.add(s, num(i, j)*2+1, LINF);
    }
    rep(k, 4) {
      int ni = i+di[k];
      int nj = j+dj[k];
      if(able(ni, nj)) {
        mf.add(num(i, j)*2+1, num(ni, nj)*2, LINF);
        mf.add(num(ni, nj)*2+1, num(i, j)*2, LINF);
      }
    }
  }

  ll u = mf.Dinic(s, t);
  if(u >= LINF) cout << -1 << endl;
  else cout << u << endl;
}

Measured memory_bytes: 9296