萌新求助WA64分

#include<bits/stdc++.h> 
#define ask(x,y) x*(n-1)+y
#define int long long
const int inf=1e15;
using namespace std;
int n,m,s,t,x,y,z,tot,maxflow,maxn,num[5000005],dp[5000005],lst[5000005],head[5000005],ver[5000005],edge[5000005],nxt[5000005],d[200005],now[5000005];
bool flag[205][205];
void add(int x,int y,int z){
    ver[++tot]=y,lst[tot]=x,edge[tot]=z,nxt[tot]=head[x],head[x]=tot;
    ver[++tot]=x,lst[tot]=y,edge[tot]=0,nxt[tot]=head[y],head[y]=tot;
}
bool bfs(){
    memset(d,0,sizeof(d));
    queue<int> q;
    q.push(s),d[s]=1,now[s]=head[s];
    while(q.size()){
        int x=q.front();
//      cout<<x<<endl;
        q.pop();
        for(int i=head[x];i;i=nxt[i]){
            int y=ver[i];
            if(edge[i]&&!d[y]){
                q.push(y);
                now[y]=head[y];
                d[y]=d[x]+1;
                if(y==t) return 1;
            }
        }
    }
    return 0;
}
int dinic(int x,int flow){
    if(x==t) return flow;
    int rest=flow;
    for(int i=now[x];i&&rest;i=nxt[i]){
        now[x]=i;
        int y=ver[i];
        if(edge[i]&&d[y]==d[x]+1){
            int k=dinic(y,min(rest,edge[i]));
            if(k==0) d[y]=0;
            edge[i]-=k,edge[i^1]+=k;
            rest-=k; 
        }
    }
    return flow-rest;
}
int dx[8]={-1,-2,2,1,-1,-2,2,1};
int dy[8]={-2,-1,-1,-2,2,1,1,2};
signed main(){
    tot=1;
    cin>>n>>m;
    s=0,t=n*n+1;
    for(int i=1;i<=m;i++){
        cin>>x>>y;
        flag[x][y]=1;
    } 
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            if(flag[i][j]) continue;
            if(((i+j)&1)==1) add(s,ask(i,j),1);
            else add(ask(i,j),t,1);
        }
    }
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            if(((i+j)&1)==0||flag[i][j]) continue;
            for(int k=0;k<8;k++){
                int fx=i+dx[k],fy=j+dy[k];
                if(fx<1||fy<1||fx>n||fy>n||flag[fx][fy]) continue;
                add(ask(i,j),ask(fx,fy),inf);
            }
        }
    }
    int flow=0;
    while(bfs()){
        while(flow=dinic(s,inf)) maxflow+=flow;
    }
    cout<<n*n-m-maxflow;
}