#ifdef LOCAL111
	#define _GLIBCXX_DEBUG
#else
	#define NDEBUG
#endif
#include <bits/stdc++.h>
const int INF = 1e9;
using namespace std;
template<typename T, typename U> ostream& operator<< (ostream& os, const pair<T,U>& p) { cout << '(' << p.first << ' ' << p.second << ')'; return os; }

#define endl '\n'
#define ALL(a)  (a).begin(),(a).end()
#define SZ(a) int((a).size())
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n)  FOR(i,0,n)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#ifdef LOCAL111
	#define DEBUG(x) cout<<#x<<": "<<(x)<<endl
	template<typename T> void dpite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;}
#else
	#define DEBUG(x) true
	template<typename T> void dpite(T a, T b){ return; }
#endif
#define F first
#define S second
#define SNP string::npos
#define WRC(hoge) cout << "Case #" << (hoge)+1 << ": "
template<typename T> void pite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;}
template<typename T> bool chmax(T& a, T b){if(a < b){a = b; return true;} return false;}
template<typename T> bool chmin(T& a, T b){if(a > b){a = b; return true;} return false;}

typedef long long int LL;
typedef unsigned long long ULL;
typedef pair<int,int> P;
typedef pair<LL,LL> LP;

#define int long long

//library
typedef long long cost_t;

class Edge {
public:
	int to, from, rev;
	cost_t cost;
	
	Edge(){
	}

	Edge(int x,cost_t y){
		to = x;
		cost = y;
	}

	Edge(int x, int y, cost_t z){
		from = x;
		to = y;
		cost = z;
	}

	Edge(int x, int y, int r, cost_t z){
		from = x;
		to = y;
		cost = z;
		rev = r;
	}

	bool operator< (const Edge& x) const {
		if(cost != x.cost) return cost < x.cost;
		return to < x.to;
	}

	bool operator> (const Edge& x) const {
		if(cost != x.cost) return cost > x.cost;
		return to > x.to;
	}
};

class Graph {
private:
	//const long long int INF = (long long)1e18;
	vector<vector<Edge> > v; 
	int n;
public:
	Graph(int x){
		n = x;
		v = vector<vector<Edge> >(x);
	}

	Graph(){}

	vector<Edge>& operator[](int x){
		return v[x];
	}

	const vector<Edge>& operator[](int x) const {
		return v[x];
	}

	int size() const {
		return n;
	}

	void add_edge(int from, Edge e){
		v[from].push_back(e);
	}

	void add_edge(int from, int to, cost_t cost, int rev = -1){
		add_edge(from,Edge(from,to,rev,cost));
	}

	void add_uedge(int from, int to, cost_t cost){
		add_edge(from,to,cost);
		add_edge(to,from,cost);
	}
};

void vizGraph(const Graph &g, bool with_dir = false, bool with_cap = false){
	ofstream ofs("./out.dot");
	if(with_dir) ofs << "graph graph_name {" << endl;
	else ofs << "digraph graph_name {" << endl;
	for(int i = 0; i < (int)g.size(); i++){
		if (!g[i].size())
			continue;
		for(const auto &e : g[i]){
			if(with_dir){
				if(e.to > i) ofs << "    \"" << i << "\" -- \"" << e.to << '"'; 
			}else ofs << "    \"" << i << "\" -> \"" << e.to << '"'; 
			
			if (with_cap) {
				ofs << " [ label = \"" << (e.cost  == INF ? "inf" : to_string(e.cost)) << "\"];"; 
			}
			if(!with_dir or e.to > i)ofs << endl;
		}
	}
	ofs << "}" << endl;
	ofs.close();
	system("dot -T png out.dot > sample.png");	
}


cost_t ffMaxFlow(const Graph &g_, int s, int t){
	Graph g = g_;
	int n = g.size();
	vector<int> rev(n);
	for(int i = 0; i < n; ++i) {
		for(int j = 0; j < (int)g[i].size(); j++) {
			auto&& e = g[i][j];
			e.rev = g[e.to].size();
			g.add_edge(e.to,i,0,j);
		}
	}

	cost_t flow = 0;
	while(true){
		vector<bool> used(n,false);

		function<cost_t(int,int,cost_t)> dfs = [&](int v, int t, cost_t f) -> cost_t{
			if(v == t) return f;
			used[v] = true;
			for(auto&& e : g[v]) {
				if(!used[e.to] and e.cost > 0){
					cost_t d = dfs(e.to, t, min(f, e.cost));
					if(d > 0){
						e.cost -= d;
						g[e.to][e.rev].cost += d;
						return d;
					}
				}
			}
			return 0;
		};

		cost_t f = dfs(s,t,INF);
		if(f == 0) return flow;
		flow += f;
	}
}

void ios_init(){
	//cout.setf(ios::fixed);
	//cout.precision(12);
#ifdef LOCAL111
	return;
#endif
	ios::sync_with_stdio(false); cin.tie(0);	
}

const int dx[] = {1,-1,0,0};
const int dy[] = {0,0,1,-1};

signed main()
{
	ios_init();
	int h,w;
	while(cin >> h >> w){
		vector<string> s(h);
		REP(i,h) cin >> s[i];
		Graph g(2*h*w+2);
		int so = h*w*2;
		int si = h*w*2+1;
		
		auto check = [&](int x, int y) -> bool{
			return 0 <= x and x < h and 0 <= y and y < w;
		};
		auto mp = [&](int x, int y) -> int{
			return w*x+y;
		};

		REP(i,h) REP(j,w){
			DEBUG(i); DEBUG(j);
			REP(k,4){
				int nx = i+dx[k];
				int ny = j+dy[k];
				DEBUG(mp(nx,ny));
				if(check(nx,ny)){
					g.add_edge(h*w+mp(i,j),mp(nx,ny),INF);
				}
			}
			if(s[i][j] == 'X'){
				g.add_edge(mp(i,j),h*w+mp(i,j),INF);
				g.add_edge(so,mp(i,j),INF);
			}else {
				g.add_edge(mp(i,j), h*w+mp(i,j),1);
			}
		}
		// vizGraph(g,false,true);
		REP(i,h){
			g.add_edge(h*w+mp(i,w-1),si,INF);
			g.add_edge(h*w+mp(i,0),si,INF);
		}
		FOR(j,1,w-1){
			g.add_edge(h*w+mp(0,j),si,INF);
			g.add_edge(h*w+mp(h-1,j),si,INF);
		}
		int ans = ffMaxFlow(g,so,si);
		if(ans >= INF){
			cout << -1 << endl;
		}else{
			cout << ans << endl;
		}
	}
	return 0;
}


Measured memory_bytes: 11256