#include <iostream>
#include <string>
#include <queue>
#include <stack>
#include <algorithm>
#include <list>
#include <vector>
#include <complex>
#include <utility>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <bitset>
#include <ctime>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <cassert>
#include <cstddef>
#include <iomanip>
#include <numeric>
#include <tuple>
#include <sstream>
#include <fstream>
#include <functional>
#include <chrono>

using namespace std;
#define REP(i, n) for (int (i) = 0; (i) < (n); (i)++)
#define FOR(i, a, b) for (int (i) = (a); (i) < (b); (i)++)
#define RREP(i, a) for(int (i) = (a) - 1; (i) >= 0; (i)--)
#define FORR(i, a, b) for(int (i) = (a) - 1; (i) >= (b); (i)--)
#define DEBUG(C) cerr << #C << " = " << C << endl;
using LL = long long;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<LL>;
using VVL = vector<VL>;
using VD = vector<double>;
using VVD = vector<VD>;
using PII = pair<int, int>;
using PDD = pair<double, double>;
using PLL = pair<LL, LL>;
using VPII = vector<PII>;
template<typename T> using VT = vector<T>;
#define ALL(a) begin((a)), end((a))
#define RALL(a) rbegin((a)), rend((a))
#define SORT(a) sort(ALL((a)))
#define RSORT(a) sort(RALL((a)))
#define REVERSE(a) reverse(ALL((a)))
#define MP make_pair
#define FORE(a, b) for (auto &&a : (b))
#define FIND(s, e) ((s).find(e) != (s).end())
#define EB emplace_back
template<typename T> inline bool chmax(T &a, T b){if (a < b){a = b;return true;}return false;}
template<typename T> inline bool chmin(T &a, T b){if (a > b){a = b;return true;}return false;}

const int INF = 1e9;
const int MOD = INF + 7;
const LL LLINF = 1e18;
const long double EPS = 1e-9;

class FordFulkerson {
    struct edge {
        int to, rev;    //行き先，逆辺(のid)
        long long cap;  //容量
        edge(int _to, int _rev, long long _cap) {
            this->to = _to;
            this->rev = _rev;
            this->cap = _cap;
        }
    };
private:
    vector<vector<edge>> G;
    vector<bool> used;
    long long dfs(int v, int t, long long f) {
        if (v == t) return f;
        used[v] = true;
        for (auto &&el : G[v]) {
            if (used[el.to] || el.cap <= 0) {
                continue;
            }
            long long d = dfs(el.to, t, min(f, el.cap));
            if (d > 0) {
                el.cap -= d;
                G[el.to][el.rev].cap += d;
                return d;
            }
        }
        return 0;
    }
public:
    FordFulkerson() {}
    FordFulkerson(int n) {
        G.resize(n);
        used.resize(n, false);
    }
    void add_edge(int from, int to, long long cap) {
        G[from].emplace_back(edge(to, G[to].size(), cap));
        G[to].emplace_back(edge(from, G[from].size() - 1, 0));
    }
    long long max_flow(int s, int t) {
        if (s == t) return 0;
        long long flow = 0;
        while (1) {
            fill(begin(used), end(used), false);
            long long f = dfs(s, t, LLINF);
            if (f == 0) return flow;
            flow += f;
        }
    }
};

const int dx[] = {0, 0, -1, 1};
const int dy[] = {1, -1, 0, 0};
int H, W;
vector<string> S;
FordFulkerson ff;

int main(void) {
    cin >> H >> W;
    S.resize(H);
    for (int i = 0; i < H; ++i) {
        cin >> S[i];
    }
    for (int i = 0; i < H; ++i) {
        if (S[i].front() == 'X' || S[i].back() == 'X') {
            puts("-1"); return 0;
        }
    }
    for (int j = 0; j < W; ++j) {
        if (S.front()[j] == 'X' || S.back()[j] == 'X') {
            puts("-1"); return 0;
        }
    }
    const int MAX = H * W * 2 + 2;//各マスをinとoutに分けるため二倍で取る（inとoutを切断する行為が柵を設置することに該当する）
    ff = FordFulkerson(MAX);
    const int s = H * W * 2;    //始点
    const int t = H * W * 2 + 1;//終点
    for (int i = 0; i < H; ++i) {
        for (int j = 0; j < W; ++j) {
            const int num = i * W + j;
            ff.add_edge(num, num + W * H, (S[i][j] == 'X' ? INF : 1));//ヤギさんがいたらそこには柵は置けません！
            if (S[i][j] == 'X') {
                assert(!(i == 0 || i == H - 1 || j == 0 || j == W - 1));
                ff.add_edge(s, num + W * H, INF);//ヤギさんを流す下準備てきな（切断不可）切断不可のoutからヤギさんにエネルギー注入
            }
            if (i == 0 || i == H - 1 || j == 0 || j == W - 1) {
                ff.add_edge(num + W * H, t, INF); //グリッドの端と外を繋げる（切断不可）
            }
            assert(num < W * H);
            for (int k = 0; k < 4; ++k) {
                const int nh = i + dx[k];
                const int nw = j + dy[k];
                if (0 <= nh && nh < H && 0 <= nw && nw < W) {
                    const int nnum = nh * W + nw;
                    assert(nnum != s && nnum != t);
                    ff.add_edge(num + W * H, nnum, INF); // inから隣接マスのoutへ接続
                }
            }
            assert(s != num && t != num);
        }
    }
    cout << ff.max_flow(s, t) << endl;
}


Measured memory_bytes: 3584