#include<bits/stdc++.h>
using namespace std;
char ch[1001][1001];
int id[1001][1001];
vector<pair<int,int>>black,white;
vector<int>e[2001];
const int dx[4]={-1,1,0,0},dy[4]={0,0,-1,1};
int connect[2001];
bool vis[1001];
inline bool match(int u)
{
    for(auto v: e[u])
    {
        if(vis[v])
            continue;
        vis[v]=true;
        if(!connect[v]||match(connect[v]))
        {
            connect[v]=u;
            return true;
        }
    }
    return false;
}
bool path[1001][1001];
int main(){
    int n,m;
    cin>>n>>m;
    int bl=0,wh=0;
    for(int i=1;i<=n;i++)
        for(int j=1;j<=m;j++)
        {
            cin>>ch[i][j];
            if(ch[i][j]=='#')
                continue;
            if((i+j)&1)
                black.push_back({i,j}),id[i][j]=(++bl);
            else
                white.push_back({i,j}),id[i][j]=(++wh);
        }
    long long bas=1ll*bl*(bl-1)/2+1ll*wh*(wh-1)/2;
    if(bas>=1000000ll)
    {
        printf("1000000\n");
        return 0;
    }
    for(auto [u,v]: black)
    {
        int x=id[u][v];
        for(int d=0;d<4;d++)
        {
            int tx=u+dx[d],ty=v+dy[d];
            if(tx<1||tx>n||ty<1||ty>m)
                continue;
            if(id[tx][ty])
            {
                int y=id[tx][ty];
                e[x].push_back(y);
            }
        }
    }
    int mat=0;
    for(int i=1;i<=bl;i++)
    {
        fill(vis+1,vis+wh+1,false);
        if(match(i))
            mat++;
    }
    assert(mat==bl);
    for(int s=1;s<=bl;s++)
    {
        fill(vis+1,vis+wh+1,false);
        auto dfs=[&](auto self,int u)->void
        {
            for(auto v: e[u])
            {
                if(vis[v])
                    continue;
                vis[v]=true;
                path[s][v]=true;
                self(self,connect[v]);
            }
        };
        dfs(dfs,s);
    }
    for(int i=1;i<=bl;i++)
        for(int j=1;j<=wh;j++)
            bas+=!path[i][j];
    bas=min(bas,1000000ll);
    printf("%lld\n",bas);
    
    return 0;
}