#include <bits/stdc++.h>
using namespace std;

using vi = vector<int>; using vvi = vector<vi>; using vvvi = vector<vvi>;
using ll = long long int;
using vll = vector<ll>; using vvll = vector<vll>; using vvvll = vector<vvll>;
using vd = vector<double>; using vvd = vector<vd>; using vvvd = vector<vvd>;
using P = pair<int, int>;
using Pll = pair<ll, ll>;
using cdouble = complex<double>;

const double eps = 1e-9;
const double INFD = numeric_limits<double>::infinity();
const double PI = 3.14159265358979323846;
#define Loop(i, n) for(int i = 0; i < (int)(n); i++)
#define Loop1(i, n) for(int i = 1; i <= (int)(n); i++)
#define Loopr(i, n) for(int i = (int)(n) - 1; i >= 0; i--)
#define Loopr1(i, n) for(int i = (int)(n); i >= 1; i--)
#define Foreach(buf, container) for(auto buf : container)
#define Loopdiag(i, j, h, w, sum) for(int i = ((sum) >= (h) ? (h) - 1 : (sum)), j = (sum) - i; i >= 0 && j < (w); i--, j++)
#define Loopdiagr(i, j, h, w, sum) for(int j = ((sum) >= (w) ? (w) - 1 : (sum)), i = (sum) - j; j >= 0 && i < (h); j--, i++)
#define Loopdiagsym(i, j, h, w, gap) for (int i = ((gap) >= 0 ? (gap) : 0), j = i - (gap); i < (h) && j < (w); i++, j++)
#define Loopdiagsymr(i, j, h, w, gap) for (int i = ((gap) > (h) - (w) - 1 ? (h) - 1 : (w) - 1 + (gap)), j = i - (gap); i >= 0 && j >= 0; i--, j--)
#define Loopitr(itr, container) for(auto itr = container.begin(); itr != container.end(); itr++)
#define printv(vector) Loop(ex_i, vector.size()) { cout << vector[ex_i] << " "; } cout << endl;
#define printmx(matrix) Loop(ex_i, matrix.size()) { Loop(ex_j, matrix[ex_i].size()) { cout << matrix[ex_i][ex_j] << " "; } cout << endl; }
#define quickio() ios::sync_with_stdio(false); cin.tie(0);
#define bitmanip(m,val) static_cast<bitset<(int)m>>(val)
#define Comp(type_t) bool operator<(const type_t &another) const
#define fst first
#define snd second
bool nearlyeq(double x, double y) { return abs(x - y) < eps; }
bool inrange(int x, int t) { return x >= 0 && x < t; }
bool inrange(vi xs, int t) { Foreach(x, xs) if (!(x >= 0 && x < t)) return false; return true; }
ll rndf(double x) { return (ll)(x + (x >= 0 ? 0.5 : -0.5)); }
ll floorsqrt(ll x) { ll m = (ll)sqrt((double)x); return m + (m * m <= x ? 0 : -1); }
ll ceilsqrt(ll x) { ll m = (ll)sqrt((double)x); return m + (x <= m * m ? 0 : 1); }
ll rnddiv(ll a, ll b) { return (a / b + (a % b * 2 >= b ? 1 : 0)); }
ll ceildiv(ll a, ll b) { return (a / b + (a % b == 0 ? 0 : 1)); }
ll gcd(ll m, ll n) { if (n == 0) return m; else return gcd(n, m % n); }
ll lcm(ll m, ll n) { return m * n / gcd(m, n); }

/*******************************************************/

typedef ll val_t;

struct graph_t {
	int n;           // |V|, index begins with 0
	int m;           // |E|
	vector<P> edges; // E
	vector<val_t> vals; // V
	vector<ll> costs; // cost or distance
	vector<ll> caps;  // capacity
};

class Maxflow {
private:
	struct edge {
		int eid, from, to;
		ll cap;
	};
	struct node {
		int id; bool done; vector<int> to_eids;
	};
	int dual_eid(int eid) {
		if (eid < m) return eid + m;
		else return eid - m;
	}
	ll dfs(int a, ll f) {
		if (a == sink) return f;
		nodes[a].done = true;
		Foreach(eid, nodes[a].to_eids) {
			int b = edges[eid].to;
			if (!nodes[b].done && edges[eid].cap > 0) {
				ll df = dfs(b, min(f, edges[eid].cap));
				if (df > 0) {
					edges[eid].cap -= df;
					edges[dual_eid(eid)].cap += df;
					return df;
				}
			}
		}
		return 0;
	}
	vector<node> nodes;
	vector<edge> edges;
	int n, m;
	int source, sink;
public:
	Maxflow(graph_t G, int s, int t) {
		n = G.n;
		m = G.edges.size();
		nodes.resize(n);
		edges.resize(m * 2);
		Loop(i, n) nodes[i] = { i, false, {} };
		Loop(i, m) {
			int a = G.edges[i].first;
			int b = G.edges[i].second;
			nodes[a].to_eids.push_back(i);
			nodes[b].to_eids.push_back(i + m);
			edges[i] = { i, a, b, G.caps[i] };
			edges[i + m] = { i + m, b, a, 0 };
		}
		source = s;
		sink = t;
		do {
			Loop(i, n) nodes[i].done = false;
		} while (dfs(source, LLONG_MAX));
		return;
	}
	vll get_eid_flow() {
		vll ret(m);
		Loop(i, m) {
			ret[i] = edges[i + m].cap;
		}
		return ret;
	}
	ll get_maxflow() {
		ll ret = 0;
		Foreach(eid, nodes[sink].to_eids) {
			if (eid >= m) ret += edges[eid].cap;
		}
		return ret;
	}
};

void failed() {
	cout << -1 << endl;
	exit(0);
}

int main() {
	int n, m; cin >> n >> m;
	vector<string> board(n);
	Loop(i, n) cin >> board[i];
	vi di = { -1,1,0,0 };
	vi dj = { 0,0,-1,1 };
	graph_t G;
	G.n = n * m * 2 + 2;
	int ofst0 = 0;
	int ofst1 = 1;
	int ofst2 = 1 + n * m;
	int ofst3 = 1 + n * m * 2;
	Loop(i, n) {
		Loop(j, m) {
			if (board[i][j] == 'X') {
				G.edges.push_back({ ofst0, ofst1 + i * m + j });
				G.caps.push_back(INT_MAX);
				G.edges.push_back({ ofst1 + i * m + j, ofst2 + i * m + j });
				G.caps.push_back(INT_MAX);
			}
			else {
				G.edges.push_back({ ofst1 + i * m + j, ofst2 + i * m + j });
				G.caps.push_back(1);
			}
			Loop(k, 4) {
				int pi = i + di[k];
				int pj = j + dj[k];
				if (inrange(pi, n) && inrange(pj, m)) {
					G.edges.push_back({ ofst2 + i * m + j, ofst1 + pi * m + pj });
					G.caps.push_back(1);
				}
				else {
					if (board[i][j] == 'X') failed();
					G.edges.push_back({ ofst2 + i * m + j, ofst3 });
					G.caps.push_back(INT_MAX);
				}
			}
		}
	}
	Maxflow mf(G, 0, ofst3);
	cout << mf.get_maxflow() << endl;
}

Measured memory_bytes: 5620