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

using pi = pair<int,int>;
#define sz(x) int((x).size())
template<class F> struct Dinic {
    struct Edge { int to, rev; F cap; };
    int N; vector<vector<Edge>> adj;
    void init(int _N) { N = _N; adj.resize(N); } //0-based
    pi ae(int a, int b, F cap, F rcap = 0) {
        assert(min(cap,rcap) >= 0); // saved me > once
        adj[a].push_back({b,sz(adj[b]),cap});
        adj[b].push_back({a,sz(adj[a])-1,rcap});
        return {a,sz(adj[a])-1};
    }
    F edgeFlow(pi loc) { // get flow along original edge
        const Edge& e = adj.at(loc.first).at(loc.second);
        return adj.at(e.to).at(e.rev).cap;
    }
    vector<int> lev, ptr;
    bool bfs(int s,int t){//level=shortest dist from source
        lev = ptr = vector<int>(N);
        lev[s] = 1; queue<int> q({s});
        while (sz(q)) { int u = q.front(); q.pop();
            for(auto& e: adj[u]) if (e.cap && !lev[e.to]) {
                q.push(e.to), lev[e.to] = lev[u]+1;
                if (e.to == t) return 1;
            }
        }
        return 0;
    }
    F dfs(int v, int t, F flo) {
        if (v == t) return flo;
        for (int& i = ptr[v]; i < sz(adj[v]); i++) {
            Edge& e = adj[v][i];
            if (lev[e.to]!=lev[v]+1||!e.cap) continue;
            if (F df = dfs(e.to,t,min(flo,e.cap))) {
                e.cap -= df; adj[e.to][e.rev].cap += df;
                return df; } // saturated $\geq1$ one edge
        }
        return 0;
    }
    F maxFlow(int s, int t) {
        F tot = 0; while (bfs(s,t)) while (F df =
            dfs(s,t,numeric_limits<F>::max())) tot += df;
        return tot;
    }
};

void solve(int t){
    int n,m;
    cin>>n>>m;
    
    vector mat(n,vector(m,'.'));
    vector ind(n,vector(m,-1));
    
    int cont=0;
    for(int i=0;i<n;i++){
        for(int j=0;j<m;j++){
            cin>>mat[i][j];
            if(mat[i][j]=='.'){
                ind[i][j]=cont++;
            }
        }
    }
    
    const int inf = 1'000'000;
    if(cont>2000)cout<<inf<<"\n";
    else{
        int ans=cont/2*(cont/2-1);
        
        vector grafo(cont,vector<int>());
        vector par(cont,false);
        
        const pair<int,int> dir[4] = {{1,0},{-1,0},{0,1},{0,-1}};
        for(int x=0;x<n;x++){
            for(int y=(x%2);y<m;y+=2){
                if(ind[x][y]>=0)par[ind[x][y]]=true;
                
                for(auto [dx,dy] : dir){
                    int sx = x+dx, sy = y+dy;
                    
                    if(sx>=0 && sx<n && sy>=0 && sy<m){
                        if(ind[x][y]>=0 && ind[sx][sy]>=0){
                            grafo[ind[x][y]].push_back(ind[sx][sy]);
                        }
                    }
                }
            }
        }
        
        auto check = [&](int x)->void{

            Dinic<int> ds;
            ds.init(cont+2);
            int source = cont, sink = cont+1;
            
            //cout<<"check "<<x<<"\n";
            for(int i=0;i<cont;i++){
                if(par[i])ds.ae(source,i,1);
                else ds.ae(i,sink,1);
            }
            
            for(int i=0;i<cont;i++){
                if(!par[i] || i==x)continue;
                for(auto j : grafo[i]){
                    //cout<<i<<"->"<<j<<"\n";
                    ds.ae(i,j,1);
                }
            }
            
            ds.maxFlow(source,sink);
            
            ans+=cont/2;
            vector vis(cont,false);
            auto dfs = [&](auto &dfs,int nodo)->void{
                if(vis[nodo])return;
                vis[nodo]=true;
                //cout<<nodo<<"\n";
                if(!par[nodo])ans--;
                
                for(auto i : ds.adj[nodo]){
                    if(i.to<cont && i.cap==0){
                        dfs(dfs,i.to);
                    }
                }
            };
            
            for(auto i : ds.adj[sink]){
                if(i.cap==0){
                    //cout<<"Inizio da "<<i.to<<"\n";
                    dfs(dfs,i.to);
                }
            }
            //cout<<ans<<" dopo "<<x<<"\n";
        };
        
        //cout<<ans<<" inizio\n";
        for(int i=0;i<n;i++){
            for(int j=0;j<m;j++){
                if(mat[i][j]=='.' && (i+j)%2==0){
                    check(ind[i][j]);
                }
                if(ans>=inf)break;
            }
            if(ans>=inf)break;
        }
        ans=min(ans,inf);
        cout<<ans<<"\n";
    }
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int t=1;
    //cin>>t;
    for(int i=1;i<=t;i++)solve(i);
    
    return 0;
}