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

const int N=505;

int n,m,num,ans;
char g[N][N];
int uid[N][N];
bool nbord[N*N];
int bordum;
vector<int> lt,rt;
vector<int> edge[N*N];

bool vis[N][N];
int dx[4]={0,0,1,-1};
int dy[4]={1,-1,0,0};
void dfs(int x,int y,int id){
    uid[x][y]=id;
    vis[x][y]=true;
    if(x==1||x==n||y==1||y==m){
        if(!nbord[id])bordum++;
        nbord[id]=true;
    }
    for(int k=0;k<4;k++){
        int nx=x+dx[k],ny=y+dy[k];
        if(nx<1||nx>n||ny<1||ny>m)continue;
        if(!vis[nx][ny]&&g[nx][ny]==g[x][y])dfs(nx,ny,id);
    }
}

bool st[N*N];
int mg[N*N];
bool hungry(int now){
    for(auto nxt:edge[now]){
        if(vis[nxt])continue;
        st[nxt]=true;
        if(!mg[nxt]||hungry(mg[nxt])){
            mg[nxt]=now;
            return true;
        }
    }
    return false;
}

int main(){
    // freopen("unity.in","r",stdin);
    // freopen("unity.out","w",stdout);
    cin>>n>>m;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=m;j++){
            cin>>g[i][j];
        }
    }
    for(int i=1;i<=n;i++){
        for(int j=1;j<=m;j++){
            if(!vis[i][j]){
                dfs(i,j,++num);
                if(nbord[num])continue;
                if(g[i][j]=='0')lt.push_back(num);
                else rt.push_back(num);
            }
        }
    }
    for(int i=1;i<=n;i++){
        for(int j=1;j<=m;j++){
            if(nbord[uid[i][j]]||g[i][j]=='1')continue;
            for(int k=0;k<4;k++){
                int nx=i+dx[k],ny=j+dy[k];
                if(uid[i][j]!=uid[nx][ny]&&!nbord[uid[nx][ny]]){
                    edge[uid[i][j]].push_back(uid[nx][ny]);
                }
            }
        }
    }
    for(int i=1;i<=num;i++){
        sort(edge[i].begin(),edge[i].end());
        auto lst=unique(edge[i].begin(),edge[i].end());
        edge[i].erase(lst,edge[i].end());
    }
    ans=bordum;
    for(auto i:lt){
        memset(st,0,sizeof(st));
        if(hungry(i))ans++;
    }
    cout<<ans;
    return 0;
}