把坐标按照(x+y)%2染色可以发现这是个二分图

二分图最大独立集=点数-最大匹配

于是就是个算匹配的傻逼题了

// luogu-judger-enable-o2

#include<bits/stdc++.h>

#define il inline

#define vd void

typedef long long ll;

il int gi(){

    int x=0,f=1;

    char ch=getchar();

    while(!isdigit(ch)){

        if(ch=='-')f=-1;

        ch=getchar();

    }

    while(isdigit(ch))x=x*10+ch-'0',ch=getchar();

    return x*f;

}

char s[210][210];

const int maxn=40010,maxm=2000000;

int num[210][210],cnt,S,T;

int fir[maxn],dis[maxm],nxt[maxm],w[maxm],id=1,head[maxn],dep[maxn];

il vd link(int a,int b){

    nxt[++id]=fir[a],fir[a]=id,dis[id]=b,w[id]=1;

    nxt[++id]=fir[b],fir[b]=id,dis[id]=a,w[id]=0;

}

const int dX[]={-1,-2,-1,-2,1,2,1,2},dY[]={2,1,-2,-1,2,1,-2,-1};

il bool BFS(){

    static int que[maxn],hd,tl;

    hd=tl=0;que[tl++]=S;

    memset(dep,0,sizeof dep);dep[S]=1;

    while(hd^tl){

        int x=que[hd++];

        for(int i=fir[x];i;i=nxt[i])

            if(w[i]&&!dep[dis[i]])que[tl++]=dis[i],dep[dis[i]]=dep[x]+1;

    }

    return dep[T];

}

il int Dinic(int x,int maxflow){

    if(x==T)return maxflow;

    int ret=0;

    for(int&i=head[x];i;i=nxt[i])

        if(w[i]&&dep[dis[i]]==dep[x]+1){

            int d=Dinic(dis[i],std::min(w[i],maxflow-ret));

            w[i]-=d,w[i^1]+=d,ret+=d;

            if(ret==maxflow)break;

        }

    return ret;

}

int main(){

#ifndef ONLINE_JUDGE

    freopen("4304.in","r",stdin);

    freopen("4303.out","w",stdout);

#endif

    int n=gi();for(int i=1;i<=n;++i)scanf("%s",s[i]+1);

    S=++cnt,T=++cnt;

    int ans=0;

    for(int i=1;i<=n;++i)

        for(int j=1;j<=n;++j){

            if(s[i][j]=='1')continue;

            num[i][j]=++cnt;++ans;

            if((i+j)&1)link(S,num[i][j]);

            else link(num[i][j],T);

        }

    for(int i=1;i<=n;++i)

        for(int j=1;j<=n;++j)

            if(s[i][j]=='0'&&((i+j)&1))

                for(int k=0;k<8;++k){

                    int xx=i+dX[k],yy=j+dY[k];

                    if(!num[xx][yy])continue;

                    link(num[i][j],num[xx][yy]);

                }

    while(BFS())memcpy(head,fir,sizeof fir),ans-=Dinic(S,1e9);

    printf("%d\n",ans);

    return 0;

}