#include <bits/stdc++.h>
#define int long long
#define endl '\n'
#define FOR(i, a, n) for (int i = (a); i < (n); ++i)
#define REP(i, n) FOR(i, 0, n)
using namespace std;

template <typename T> class Dinic {
    struct edge {
        int to, rev;
        T cap;
        bool isrev;
        edge(int t, T c, int r, bool i) : to(t), cap(c), rev(r), isrev(i) {}
    };

    void bfs(int s, int t) {
        lev.assign(g.size(), -1);
        queue<int> que;
        lev[s] = 0;
        que.emplace(s);
        while (!que.empty() && lev[t] == -1) {
            int v = que.front();
            que.pop();
            for (edge& e : g[v]) {
                if (lev[e.to] == -1 && e.cap > 0) {
                    lev[e.to] = lev[v] + 1;
                    que.emplace(e.to);
                }
            }
        }
    }

    T dfs(int v, int t, T f) {
        if (v == t) return f;
        for (int& i = ite[v]; i < g[v].size(); ++i) {
            edge& e = g[v][i];
            if (e.cap > 0 && lev[v] < lev[e.to]) {
                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;
    }

public:

    const T INF;
    vector<int> lev, ite;
    vector<vector<edge>> g;

    Dinic(int n) : INF(numeric_limits<T>::max()), g(n) {}

    void add_edge(int f, int t, T c = numeric_limits<T>::max()) {
        g[f].emplace_back(t, c, g[t].size(), false);
        g[t].emplace_back(f, 0, g[f].size() - 1, true);
    }

    T max_flow(int s, int t) {
        T res = 0;
        while (1) {
            bfs(s, t);
            if (lev[t] < 0) break;
            ite.assign(g.size(), 0);
            T f = 0;
            while ((f = dfs(s, t, INF)) > 0) {
                res += f;
            }
        }
        return res;
    }

    void print() {
        for (int i = 0; i < g.size(); ++i) {
            for (auto& e : g[i]) {
                if (e.isrev) continue;
                auto& rev_e = g[e.to][e.rev];
                cout << i << " -> " << e.to << " (flow: " << rev_e.cap << "/" << e.cap + rev_e.cap << ")" << endl;
            }
        }
    }
};

const int dy[] = {-1, 0, 0, 1};
const int dx[] = {0, -1, 1, 0};
int H, W;
string S[100];

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> H >> W;
    REP (i, H) cin >> S[i];
    REP (i, H) if (S[i][0] == 'X' || S[i][W - 1] == 'X') {
        cout << -1 << endl;
        return 0;
    }
    REP (j, W) if (S[0][j] == 'X' || S[H - 1][j] == 'X') {
        cout << -1 << endl;
        return 0;
    }
    Dinic<int> g(2 * H * W + 2);
    int s = 2 * H * W, t = s + 1;
    REP (i, H) REP (j, W) {
        if (S[i][j] == 'X') {
            g.add_edge(s, 2 * (i * W + j) + 1);
            g.add_edge(2 * (i * W + j), 2 * (i * W + j) + 1);
        } else {
            g.add_edge(2 * (i * W + j), 2 * (i * W + j) + 1, 1);
        }
        REP (k, 4) {
            int ny = i + dy[k];
            int nx = j + dx[k];
            if (ny >= 0 && ny < H && nx >= 0 && nx < W) {
                g.add_edge(2 * (i * W + j) + 1, 2 * (ny * W + nx));
            }
        }
    }
    REP (i, H) {
        g.add_edge(2 * (i * W) + 1, t);
        g.add_edge(2 * (i * W + W - 1) + 1, t);
    }
    REP (j, W) {
        g.add_edge(2 * j + 1, t);
        g.add_edge(2 * ((H - 1) * W + j) + 1, t);
    }
    cout << g.max_flow(s, t) << endl;
}

Measured memory_bytes: 6144