//四則演算 #pragma GCC target("avx")
//並列計算 #pragma GCC optimize("O3")
//条件分岐を減らす #pragma GCC optimize("unroll-loops")
//浮動小数点演算 #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using VI = vector<int>;
using VL = vector<ll>;
using VS = vector<string>;
template<class T> using PQ = priority_queue<T, vector<T>, greater<T>>;
#define FOR(i,a,n) for(int i=(a);i<(n);++i)
#define eFOR(i,a,n) for(int i=(a);i<=(n);++i)
#define rFOR(i,a,n) for(int i=(n)-1;i>=(a);--i)
#define erFOR(i,a,n) for(int i=(n);i>=(a);--i)
#define each(i, a) for(auto &i : a)
#define SORT(a) sort(a.begin(),a.end())
#define rSORT(a) sort(a.rbegin(),a.rend())
#define fSORT(a,f) sort(a.begin(),a.end(),f)
#define all(a) a.begin(),a.end()
#define out(y,x) ((y)<0||h<=(y)||(x)<0||w<=(x))
#define tp(a,i) get<i>(a)
#define line cout << "-----------------------------\n" 
#define ENDL(i,n) ((i) == (n) - 1 ? "\n" : " ")
#define stop system("pause")
constexpr ll INF = 1000000000;
constexpr ll LLINF = 1LL << 60;
constexpr ll mod = 1000000007;
constexpr ll MOD = 998244353;
constexpr ld eps = 1e-10;
constexpr ld pi = 3.1415926535897932;
template<class T>inline bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; }return false; }
template<class T>inline bool chmin(T& a, const T& b) { if (a > b) { a = b; return true; }return false; }
inline void init() { cin.tie(nullptr); cout.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); }
template<class T>inline istream& operator>>(istream& is, vector<T>& v) { for (auto& a : v)is >> a; return is; }
template<class T>inline istream& operator>>(istream& is, deque<T>& v) { for (auto& a : v)is >> a; return is; }
template<class T, class U>inline istream& operator>>(istream& is, pair<T, U>& p) { is >> p.first >> p.second; return is; }
template<class T>inline vector<T> vec(size_t a) { return vector<T>(a); }
template<class T>inline vector<T> defvec(T def, size_t a) { return vector<T>(a, def); }
template<class T, class... Ts>inline auto vec(size_t a, Ts... ts) { return vector<decltype(vec<T>(ts...))>(a, vec<T>(ts...)); }
template<class T, class... Ts>inline auto defvec(T def, size_t a, Ts... ts) { return vector<decltype(defvec<T>(def, ts...))>(a, defvec<T>(def, ts...)); }
template<class T>inline void print(const T& a) { cout << a << "\n"; }
template<class T, class... Ts>inline void print(const T& a, const Ts&... ts) { cout << a << " "; print(ts...); }
template<class T>inline void print(const vector<T>& v) { for (int i = 0; i < v.size(); ++i)cout << v[i] << (i == v.size() - 1 ? "\n" : " "); }
template<class T>inline void print(const vector<vector<T>>& v) { for (auto& a : v)print(a); }
inline string reversed(const string& s) { string t = s; reverse(all(t)); return t; }

int main() {
    init();

    int h, w; cin >> h >> w;
    VS field(h); cin >> field;
    {
        bool ng = false;
        FOR(y, 0, h)ng |= field[y][0] == 'X' || field[y][w - 1] == 'X';
        FOR(x, 0, w)ng |= field[0][x] == 'X' || field[h - 1][x] == 'X';
        if (ng) {
            print(-1);
            return 0;
        }
    }

    int s = h * w * 2, t = h * w * 2 + 1;
    vector<vector<tuple<int, int, int>>> g(h * w * 2 + 2);
    constexpr int dy[] = { -1,0,0,1 }, dx[] = { 0,-1,1,0 };
    FOR(y, 0, h)FOR(x, 0, w) {
        int cur_in = (y * w + x) * 2, cur_out = cur_in + 1;
        if (y % (h - 1) == 0 || x % (w - 1) == 0) {
            g[cur_out].emplace_back(t, INF, g[t].size());
            g[t].emplace_back(cur_out, 0, g[cur_out].size() - 1);
        }
        int cap = 1;
        if (field[y][x] == 'X') {
            cap = INF;
            g[s].emplace_back(cur_out, INF, g[cur_out].size());
            g[cur_out].emplace_back(s, 0, g[s].size() - 1);
        }
        g[cur_in].emplace_back(cur_out, cap, g[cur_out].size());
        g[cur_out].emplace_back(cur_in, 0, g[cur_in].size() - 1);
        FOR(i, 0, 4) {
            int ny = y + dy[i], nx = x + dx[i];
            if (out(ny, nx))continue;
            g[cur_out].emplace_back((ny * w + nx) * 2, INF, g[(ny * w + nx) * 2].size());
            g[(ny * w + nx) * 2].emplace_back(cur_out, 0, g[cur_out].size() - 1);
        }
    }

    vector<bool> used;
    auto dfs = [&](auto&& f, int cur)->bool {
        used[cur] = true;
        if (cur == t)return true;
        each(dest, g[cur]) {
            auto [to, cap, rev] = dest;
            if (used[to] || cap == 0)continue;
            if (f(f, to)) {
                --tp(dest, 1);
                ++tp(g[to][rev], 1);
                return true;
            }
        }
        return false;
    };

    int ans = 0;
    while (1) {
        used.assign(h* w * 2 + 2, false);
        if (!dfs(dfs, s))break;
        ++ans;
    }

    print(ans);

    return 0;
}

Measured memory_bytes: 6716