最多可选多少景点

#include<cstdio>#include<iostream>#include<cstring>#define LL long long#define inf 0x7ffffff#define S 0#define T 2*n+1using namespace std;inline LL read(){    LL x=0,f=1;char ch=getchar();    while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}    while(ch>=‘0‘&&ch<=‘9‘){x=x*10+ch-‘0‘;ch=getchar();}    return x*f;}struct edge{int to,next,v;}e[100010];int n,m,cnt=1,ans;bool go[1010][1010];int head[100010];int q[100010];int h[100010];inline void ins(int u,int v,int w){    e[++cnt].to=v;    e[cnt].v=w;    e[cnt].next=head[u];    head[u]=cnt;}inline void insert(int u,int v,int w){    ins(u,v,w);    ins(v,u,0);}inline bool bfs(){    memset(h,-1,sizeof(h));    q[1]=S;h[S]=0;int t=0,w=1;    while (t<w)    {        int now=q[++t];        for (int i=head[now];i;i=e[i].next)          if (e[i].v&&h[e[i].to]==-1)          {            q[++w]=e[i].to;            h[e[i].to]=h[now]+1;          }    }    if (h[T]==-1)return 0;    return 1;}inline int dfs(int x,int f){    if (x==T||!f)return f;    int w,used=0;    for (int i=head[x];i;i=e[i].next)        if (e[i].v&&h[e[i].to]==h[x]+1)        {            w=dfs(e[i].to,min(f-used,e[i].v));            e[i].v-=w;            e[i^1].v+=w;            used+=w;            if (used==f)return f;        }    if (!used)h[x]=-1;    return used;}inline void dinic(){while (bfs())ans+=dfs(S,inf);}int main(){    n=read();m=read();    for (int i=1;i<=m;i++)    {        int x=read(),y=read();        go[x][y]=1;    }    for (int k=1;k<=n;k++)    for (int i=1;i<=n;i++)    for (int j=1;j<=n;j++)    if (go[i][k]&&go[k][j])go[i][j]=1;         for(int i=1;i<=n;i++)    for(int j=1;j<=n;j++)    if (go[i][j])insert(i,j+n,1);         for (int i=1;i<=n;i++)    insert(S,i,1),insert(i+n,T,1);    dinic();    printf("%d\n",n-ans);}