#if __INCLUDE_LEVEL__ == 0

#include __BASE_FILE__

namespace {

void solve() {
  int h, w;
  scan(h, w);
  std::vector<std::string> s(h);
  scan(s);

  std::array<int, 2> n{};
  std::vector id(h, std::vector<int>(w, -1));
  for (const int i : rep(h)) {
    for (const int j : rep(w)) {
      if (s[i][j] == '.') {
        id[i][j] = n[(i + j) % 2]++;
      }
    }
  }

  std::vector<std::pair<int, int>> edges;
  for (const int i : rep(h)) {
    for (const int j : rep(w)) {
      if (s[i][j] != '.') {
        continue;
      }
      if (i + 1 < h && s[i + 1][j] == '.') {
        if ((i + j) % 2 == 0) {
          edges.emplace_back(id[i][j], id[i + 1][j]);
        } else {
          edges.emplace_back(id[i + 1][j], id[i][j]);
        }
      }
      if (j + 1 < w && s[i][j + 1] == '.') {
        if ((i + j) % 2 == 0) {
          edges.emplace_back(id[i][j], id[i][j + 1]);
        } else {
          edges.emplace_back(id[i][j + 1], id[i][j]);
        }
      }
    }
  }

  const auto dm = dulmage_mendelsohn(n[0], n[1], edges);
  const int ans = len(dm[0].second) + len(dm.back().first);
  print(ans);
}

}  // namespace

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  std::cout << std::setprecision(DBL_DECIMAL_DIG);

  solve();
}

#else  // __INCLUDE_LEVEL__

#include <bits/stdc++.h>

// https://hitonanode.github.io/cplib-cpp/graph/dulmage_mendelsohn_decomposition.hpp.html

// https://ei1333.github.io/luzhiled/snippets/graph/hopcroft-karp.html
struct BipartiteMatching {
  int V;
  std::vector<std::vector<int>> to;
  std::vector<int> dist;
  std::vector<int> match;
  std::vector<int> used, vv;
  std::vector<int> color;

  BipartiteMatching() = default;
  BipartiteMatching(int V_) : V(V_), to(V_), match(V_, -1), used(V_), color(V_, -1) {}

  void add_edge(int u, int v) {
    assert(u >= 0 and u < V and v >= 0 and v < V and u != v);
    to[u].push_back(v);
    to[v].push_back(u);
  }

  void _bfs() {
    dist.assign(V, -1);
    std::vector<int> q;
    int lq = 0;
    for (int i = 0; i < V; i++) {
      if (!color[i] and !used[i]) q.push_back(i), dist[i] = 0;
    }

    while (lq < int(q.size())) {
      int now = q[lq++];
      for (auto nxt : to[now]) {
        int c = match[nxt];
        if (c >= 0 and dist[c] == -1) q.push_back(c), dist[c] = dist[now] + 1;
      }
    }
  }

  bool _dfs(int now) {
    vv[now] = true;
    for (auto nxt : to[now]) {
      int c = match[nxt];
      if (c < 0 or (!vv[c] and dist[c] == dist[now] + 1 and _dfs(c))) {
        match[nxt] = now, match[now] = nxt;
        used[now] = true;
        return true;
      }
    }
    return false;
  }

  bool _color_bfs(int root) {
    color[root] = 0;
    std::vector<int> q{root};
    int lq = 0;
    while (lq < int(q.size())) {
      int now = q[lq++], c = color[now];
      for (auto nxt : to[now]) {
        if (color[nxt] == -1) {
          color[nxt] = !c, q.push_back(nxt);
        } else if (color[nxt] == c) {
          return false;
        }
      }
    }
    return true;
  }

  int solve() {
    for (int i = 0; i < V; i++) {
      if (color[i] == -1 and !_color_bfs(i)) return -1;
    }
    int ret = 0;
    while (true) {
      _bfs();
      vv.assign(V, false);
      int flow = 0;
      for (int i = 0; i < V; i++) {
        if (!color[i] and !used[i] and _dfs(i)) flow++;
      }
      if (!flow) break;
      ret += flow;
    }
    return ret;
  }

  template <class OStream>
  friend OStream& operator<<(OStream& os, const BipartiteMatching& bm) {
    os << "{N=" << bm.V << ':';
    for (int i = 0; i < bm.V; i++) {
      if (bm.match[i] > i) os << '(' << i << '-' << bm.match[i] << "),";
    }
    return os << '}';
  }
};

struct DirectedGraphSCC {
  int V;
  std::vector<std::vector<int>> to, from;
  std::vector<int> used;
  std::vector<int> vs;
  std::vector<int> cmp;
  int scc_num = -1;

  DirectedGraphSCC(int V = 0) : V(V), to(V), from(V), cmp(V) {}

  void _dfs(int v) {
    used[v] = true;
    for (auto t : to[v])
      if (!used[t]) _dfs(t);
    vs.push_back(v);
  }
  void _rdfs(int v, int k) {
    used[v] = true;
    cmp[v] = k;
    for (auto t : from[v])
      if (!used[t]) _rdfs(t, k);
  }

  void add_edge(int from_, int to_) {
    assert(from_ >= 0 and from_ < V and to_ >= 0 and to_ < V);
    to[from_].push_back(to_);
    from[to_].push_back(from_);
  }

  int FindStronglyConnectedComponents() {
    used.assign(V, false);
    vs.clear();
    for (int v = 0; v < V; v++)
      if (!used[v]) _dfs(v);
    used.assign(V, false);
    scc_num = 0;
    for (int i = (int)vs.size() - 1; i >= 0; i--)
      if (!used[vs[i]]) _rdfs(vs[i], scc_num++);
    return scc_num;
  }

  int _c, _init;
  std::vector<int> _ret_cycle;
  bool _dfs_detectcycle(int now, bool b0) {
    if (now == _init and b0) return true;
    for (auto nxt : to[now])
      if (cmp[nxt] == _c and !used[nxt]) {
        _ret_cycle.emplace_back(nxt), used[nxt] = 1;
        if (_dfs_detectcycle(nxt, true)) return true;
        _ret_cycle.pop_back();
      }
    return false;
  }
  std::vector<int> DetectCycle() {
    int ns = FindStronglyConnectedComponents();
    if (ns == V) return {};
    std::vector<int> cnt(ns);
    for (auto x : cmp) cnt[x]++;
    _c = std::find_if(cnt.begin(), cnt.end(), [](int x) { return x > 1; }) - cnt.begin();
    _init = std::find(cmp.begin(), cmp.end(), _c) - cmp.begin();
    used.assign(V, false);
    _ret_cycle.clear();
    _dfs_detectcycle(_init, false);
    return _ret_cycle;
  }

  DirectedGraphSCC GenerateTopologicalGraph() {
    DirectedGraphSCC newgraph(scc_num);
    for (int s = 0; s < V; s++)
      for (auto t : to[s]) {
        if (cmp[s] != cmp[t]) newgraph.add_edge(cmp[s], cmp[t]);
      }
    return newgraph;
  }
};

struct SATSolver : DirectedGraphSCC {
  int nb_sat_vars;
  std::vector<int> solution;
  SATSolver(int nb_variables = 0)
      : DirectedGraphSCC(nb_variables * 2), nb_sat_vars(nb_variables), solution(nb_sat_vars) {}
  void add_x_or_y_constraint(bool is_x_true, int x, bool is_y_true, int y) {
    assert(x >= 0 and x < nb_sat_vars);
    assert(y >= 0 and y < nb_sat_vars);
    if (!is_x_true) x += nb_sat_vars;
    if (!is_y_true) y += nb_sat_vars;
    add_edge((x + nb_sat_vars) % (nb_sat_vars * 2), y);
    add_edge((y + nb_sat_vars) % (nb_sat_vars * 2), x);
  }
  bool run() {
    FindStronglyConnectedComponents();
    for (int i = 0; i < nb_sat_vars; i++) {
      if (cmp[i] == cmp[i + nb_sat_vars]) return false;
      solution[i] = cmp[i] > cmp[i + nb_sat_vars];
    }
    return true;
  }
};

std::vector<std::pair<std::vector<int>, std::vector<int>>> dulmage_mendelsohn(
    int L, int R, const std::vector<std::pair<int, int>>& edges) {
  for (auto p : edges) {
    assert(0 <= p.first and p.first < L);
    assert(0 <= p.second and p.second < R);
  }

  BipartiteMatching bm(L + R);
  for (auto p : edges) bm.add_edge(p.first, L + p.second);
  bm.solve();

  DirectedGraphSCC scc(L + R);
  for (auto p : edges) scc.add_edge(p.first, L + p.second);
  for (int l = 0; l < L; ++l) {
    if (bm.match[l] >= L) scc.add_edge(bm.match[l], l);
  }

  int nscc = scc.FindStronglyConnectedComponents();
  std::vector<int> cmp_map(nscc, -2);

  std::vector<int> vis(L + R);
  std::vector<int> st;
  for (int c = 0; c < 2; ++c) {
    std::vector<std::vector<int>> to(L + R);
    auto color = [&L](int x) { return x >= L; };
    for (auto p : edges) {
      int u = p.first, v = L + p.second;
      if (color(u) != c) std::swap(u, v);
      to[u].push_back(v);
      if (bm.match[u] == v) to[v].push_back(u);
    }
    for (int i = 0; i < L + R; ++i) {
      if (bm.match[i] >= 0 or color(i) != c or vis[i]) continue;
      vis[i] = 1, st = {i};
      while (!st.empty()) {
        int now = st.back();
        cmp_map[scc.cmp[now]] = c - 1;
        st.pop_back();
        for (int nxt : to[now]) {
          if (!vis[nxt]) vis[nxt] = 1, st.push_back(nxt);
        }
      }
    }
  }

  int nset = 1;
  for (int n = 0; n < nscc; ++n) {
    if (cmp_map[n] == -2) cmp_map[n] = nset++;
  }
  for (auto& x : cmp_map) {
    if (x == -1) x = nset;
  }
  nset++;

  std::vector<std::pair<std::vector<int>, std::vector<int>>> groups(nset);

  for (int l = 0; l < L; ++l) {
    if (bm.match[l] < 0) continue;
    int c = cmp_map[scc.cmp[l]];
    groups[c].first.push_back(l);
    groups[c].second.push_back(bm.match[l] - L);
  }
  for (int l = 0; l < L; ++l) {
    if (bm.match[l] >= 0) continue;
    int c = cmp_map[scc.cmp[l]];
    groups[c].first.push_back(l);
  }
  for (int r = 0; r < R; ++r) {
    if (bm.match[L + r] >= 0) continue;
    int c = cmp_map[scc.cmp[L + r]];
    groups[c].second.push_back(r);
  }

  return groups;
}

template <class T, class U = T>
bool chmin(T& x, U&& y) {
  return y < x && (x = std::forward<U>(y), true);
}

template <class T, class U = T>
bool chmax(T& x, U&& y) {
  return x < y && (x = std::forward<U>(y), true);
}

template <std::signed_integral T = int>
T inf() {
  T ret;
  std::memset(&ret, 0x3f, sizeof(ret));
  return ret;
}

template <std::floating_point T>
T inf() {
  return std::numeric_limits<T>::infinity();
}

template <class T>
concept Range = std::ranges::range<T> && !std::convertible_to<T, std::string_view>;

template <class T>
concept Tuple = std::__is_tuple_like<T>::value && !Range<T>;

namespace std {

istream& operator>>(istream& is, Range auto&& r) {
  for (auto&& e : r) {
    is >> e;
  }
  return is;
}

istream& operator>>(istream& is, Tuple auto&& t) {
  return apply([&](auto&... xs) -> istream& { return (is >> ... >> xs); }, t);
}

ostream& operator<<(ostream& os, Range auto&& r) {
  for (string_view sep = ""; auto&& e : r) {
    os << exchange(sep, " ") << e;
  }
  return os;
}

ostream& operator<<(ostream& os, Tuple auto&& t) {
  const auto f = [&](auto&... xs) -> ostream& {
    [[maybe_unused]] string_view sep = "";
    ((os << exchange(sep, " ") << xs), ...);
    return os;
  };
  return apply(f, t);
}

}  // namespace std

void scan(auto&&... xs) { std::cin >> std::tie(xs...); }
void print(auto&&... xs) { std::cout << std::tie(xs...) << '\n'; }

template <class F>
class fix {
 public:
  explicit fix(F f) : f_(std::move(f)) {}

  decltype(auto) operator()(auto&&... xs) const {
    return f_(std::ref(*this), std::forward<decltype(xs)>(xs)...);
  }

 private:
  F f_;
};

inline auto rep(int l, int r) { return std::views::iota(std::min(l, r), r); }
inline auto rep(int n) { return rep(0, n); }
inline auto rep1(int l, int r) { return rep(l, r + 1); }
inline auto rep1(int n) { return rep(1, n + 1); }

namespace ranges = std::ranges;
namespace views = std::views;

using i64 = std::int64_t;

#define len(...) static_cast<int>(ranges::size(__VA_ARGS__))

#endif  // __INCLUDE_LEVEL__