#pragma GCC optimize("Ofast")
//#pragma GCC target("avx,avx2,fma")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx")
#include <bits/stdc++.h>
#define rep(i,n) for(int i=1;i<=n;++i)
#define pb push_back
#define mp make_pair
using namespace std;
const int maxn = 3000 + 5; // 单侧顶点的最大数目
// 二分图最大基数匹配
struct BPM {
  int n, m;               // 左右顶点个数
  vector<int> G[maxn];    // 邻接表
  int left[maxn];         // left[i]为右边第i个点的匹配点编号，-1表示不存在
  bool T[maxn];           // T[i]为右边第i个点是否已标记

  int right[maxn];        // 求最小覆盖用
  bool S[maxn];           // 求最小覆盖用

  void init(int n, int m) {
    this->n = n;
    this->m = m;
    for(int i = 0; i < n; i++) G[i].clear();
  }

  void AddEdge(int u, int v) {
    G[u].push_back(v);
    //cout<<u<<" "<<v<<endl;
  }

  bool match(int u){
    S[u] = true;
    for(int i = 0; i < G[u].size(); i++) {
      int v = G[u][i];
      if (!T[v]){
        T[v] = true;
        if (left[v] == -1 || match(left[v])){
          left[v] = u;
          right[u] = v;
          return true;
        }
      }
    }
    return false;
  }

  // 求最大匹配
  int solve() {
    memset(left, -1, sizeof(left));
    memset(right, -1, sizeof(right));
    int ans = 0;
    for(int u = 0; u < n; u++) { // 从左边结点u开始增广
      memset(S, 0, sizeof(S));
      memset(T, 0, sizeof(T));
      if(match(u)) ans++;
    }
    return ans;
  }
  int v[maxn];
    void dfs(int x)
    {
        if (v[x])return;
        v[x]=1;
        for(auto p:G[x])
        if (left[p]!=-1)dfs(left[p]);
    }
  int work()
  {
    for(int i=0;i<n;i++)v[i]=0;
    for(int i=0;i<n;i++)
        if (right[i]==-1)dfs(i);
    int tot=0;
    for(int i=0;i<n;i++)
        if (!v[i])tot++;
    return tot;
  }
}G;
char s[110][110];
int id[110][110];
int n,m;
const int dx[4]={0,0,-1,1};
const int dy[4]={-1,1,0,0};
int work(int sign)
{
    int tot1=0,tot2=0;
    rep(i,n)rep(j,m)
    {
        if (s[i][j]=='#')continue;
        if ((i+j)%2==sign)id[i][j]=tot1++; else id[i][j]=tot2++;
    }
    G.init(tot1,tot2);
    rep(i,n)rep(j,m)
    {
        if (s[i][j]=='#')continue;
        if ((i+j)%2!=sign)continue;
        for(int p=0;p<4;p++)
        {
            int ti=i+dx[p],tj=j+dy[p];
            if (ti<=0 ||ti>n ||tj<=0||tj>m)continue;
            if (s[ti][tj]=='#')continue;
            G.AddEdge(id[i][j],id[ti][tj]);
        }
    }
    G.solve();
    return tot1-G.work();
}
int main()
{
    cin>>n>>m;
    rep(i,n)scanf("%s",s[i]+1);
    //cout<<work(0)<<endl;
    cout<<work(0)+work(1)<<endl;
    return 0;
}