#include<bits/stdc++.h>
using namespace std;
int main()
{
	ios_base::sync_with_stdio(false);
	int n,m;
	cin>>n>>m;
	vector<string> s(n+5);
	for(int i=1;i<=n;i++)
	{
		cin>>s[i];
		s[i]=' '+s[i];
	}
	vector<int> dx={1,0,-1,0},dy={0,1,0,-1};
	auto inb=[&](int x,int y){return 1<=x and x<=n and 1<=y and y<=m;};
	auto hs=[&](int x,int y){return (x-1)*m+y;};
	vector<vector<int>> G(n*m+5);
	vector<int> ty(n*m+5),ma(n*m+5),vis(n*m+5);
	for(int i=1;i<=n;i++)
	{
		for(int j=1;j<=m;j++)
		{
			ty[hs(i,j)]=-1;
			if(s[i][j]=='#')continue;
			ty[hs(i,j)]=(i+j)%2;
			for(int d=0;d<4;d++)
			{
				int x=i+dx[d],y=j+dy[d];
				if(inb(x,y) and s[x][y]!='#')
				{
					G[hs(i,j)].push_back(hs(x,y));
					G[hs(x,y)].push_back(hs(i,j));
				}
			}
		}
	}
	for(int u=1;u<=n*m;u++)
	{
		if(ty[u]==0)
		{
			for(auto v:G[u])
			{
				if(ma[v]==0)
				{
					ma[v]=u;
					ma[u]=v;
					break;
				}
			}
		}
	}
	function<bool(int)> dfs=[&](int u)
	{
		vis[u]=1;
		for(auto v:G[u])
		{
			if(not ma[v] or (not vis[ma[v]] and dfs(ma[v])))
			{
				ma[v]=u;
				ma[u]=v;
				return true;
			}
		}
		return false;
	};
	for(int u=1;u<=n*m;u++)
	{
		if(ty[u]==0 and not ma[u])
		{
			fill(vis.begin(),vis.end(),0);
			dfs(u);
		}
	}
	fill(vis.begin(),vis.end(),0);
	for(int u=1;u<=n*m;u++)
	{
		if(ty[u]!=-1 and not ma[u])
		{
			dfs(u);
		}
	}
	int ans=0;
	for(int u=1;u<=n*m;u++)
	{
//		cerr<<u<<' '<<ma[u]<<' '<<vis[u]<<endl;
		if(ty[u]!=-1 and (not ma[u] or vis[u]))
			ans++;
	}
	cout<<ans<<endl;
	return 0;
}