#include <map>
#include <set>
#include <array>
#include <cmath>
#include <deque>
#include <bitset>
#include <random>
#include <string>
#include <vector>
#include <cassert>
#include <complex>
#include <iomanip>
#include <iostream>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>

using namespace std;

const int SZ = 100;
string arr[SZ];

const int MAXN = 3e3;
vector<int> g[MAXN];
int bad[MAXN];
int p[MAXN], in_m[MAXN];
int uid = 0;
int used[MAXN];

bool dfs(int v) {
    /// cerr << "v: " << v << endl;
    used[v] = uid;
    for (int u : g[v]) {
        /// cerr << v << "->" << u << endl;
        if (p[u] == -1 || (used[p[u]] != uid && dfs(p[u]))) {
            p[u] = v;
            p[v] = u;
            in_m[u] = in_m[v] = 1;
            return true;
        }
    }
    return false;
}

int col[MAXN];

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    int n, m;
    cin >> n >> m;
    for (int i = 0; i < n; ++i) {
        cin >> arr[i];
    }
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {
            col[i * m + j] = (i + j) % 2;
            if (arr[i][j] == '#') {
                bad[i * m + j] = 1;
                continue;
            }
            if (i > 0 && arr[i - 1][j] == '.') {
                g[i * m + j].push_back((i - 1) * m + j);
                g[(i - 1) * m + j].push_back(i * m + j);
            }
            if (j > 0 && arr[i][j - 1] == '.') {
                g[i * m + j].push_back(i * m + j - 1);
                g[i * m + j - 1].push_back(i * m + j);
            }
        }
    }
    fill(p, p + n * m, -1);

    bool found = true;
    while (found) {
        found = false;
        ++uid;
        for (int i = 0; i < n * m; ++i) {
            if (col[i] == 1 || bad[i]) continue;
            if (!in_m[i] && used[i] != uid) {
                found |= dfs(i);
            }
        } 
    }

    ++uid;
    for (int i = 0; i < n * m; ++i) {
        if (bad[i]) continue;
        if (!in_m[i] && used[i] != uid) {
            dfs(i);
        }
    }

    int cnt = 0;
    for (int i = 0; i < n * m; ++i) {
        if (used[i] == uid) {
            /// cerr << i << endl;
            ++cnt;
        }
    }
    cout << cnt << "\n";
    
    return 0;
}