#include <bits/stdc++.h>
using namespace std;
#define pb push_back

const int N=55;
const int inf=1e9+7;
const int M=N*N;
vector<int> G[M];
struct Edge{
	int u,v,c,f;
};
vector<Edge> edges;
void AddEdge(int u,int v,int c){
	G[u].pb(edges.size());
	edges.pb(Edge{u,v,c,0});
	G[v].pb(edges.size());
	edges.pb(Edge{v,u,0,0});
}
int dist[M],s,t,ptr[M];
bool BFS(){
	for(int i=s;i<=t;i++)dist[i]=-1,ptr[i]=0;
	queue<int> q;
	q.push(s);
	dist[s]=0;
	while(q.size()){
		int u=q.front();
		q.pop();
		for(int e:G[u]){
			int v=edges[e].v;
			int c=edges[e].c-edges[e].f;
			if(c>0 && dist[v]==-1){
				dist[v]=dist[u]+1;
				q.push(v);
			}
		}
	}
	return dist[t]!=-1;
}
int DFS(int u,int flow){
	if(u==t)return flow;
	if(dist[u]>=dist[t])return 0;
	int ans=0;
	for(;ptr[u]<G[u].size();ptr[u]++){
		int e=G[u][ptr[u]];
		int v=edges[e].v;
		int c=edges[e].c-edges[e].f;
		if(c==0 || dist[v]!=dist[u]+1)continue;
		int take=DFS(v,min(flow,c));
		edges[e].f+=take;
		edges[e^1].f-=take;
		ans+=take;
		flow-=take;
		if(!flow)break;
	}
	return ans;
}
char grid[N][N];
int mv[2][2]={{0,1},{1,0}},idx[N][N];
int MaxFlow(int n,int m){
	s=0;t=n*m+1;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=m;j++){
			if(grid[i][j]=='#')continue;
			if((i+j)%2==0)AddEdge(s,idx[i][j],1);
			else AddEdge(idx[i][j],t,1);
			for(int k=0;k<2;k++){
				int x=i+mv[k][0];
				int y=j+mv[k][1];
				if(x>=1 && x<=n && y>=1 && y<=m && grid[x][y]=='.'){
					if((i+j)%2==0)AddEdge(idx[i][j],idx[x][y],1);
					else AddEdge(idx[x][y],idx[i][j],1);
				}
			}
		}
	}
	int ans=0;
	while(BFS()){
		ans+=DFS(s,inf);
	}
	edges.clear();
	for(int i=s;i<=t;i++)G[i].clear();
	return ans;
}

int main(){
	int n,m;
	scanf("%i %i",&n,&m);
	for(int i=1;i<=n;i++){
		scanf("%s",grid[i]+1);
		for(int j=1;j<=m;j++){
			idx[i][j]=(i-1)*m+j;
		}
	}
	int mx=MaxFlow(n,m),ans=0;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=m;j++){
			if(grid[i][j]=='.'){
				grid[i][j]='#';
				if(MaxFlow(n,m)==mx)ans++;
				grid[i][j]='.';
			}
		}
	}
	printf("%i\n",ans);
	return 0;
}