Code:

#include<algorithm>
#include<cstdio>               
#include<algorithm>
#include<cstring>
using namespace std;
typedef long long ll;
const int maxn=1000000+233;
const int N=1000000+233;
int A[N],B[N];                 
ll G[N],F[N];                  
int head[maxn*2],nex[maxn*
 
2],to[maxn*2],val[maxn],vis[maxn];
int cnt,num,root;
struct Circle{
    int p[maxn];
    void init(){
        for(int i=1;i<maxn-12;++i)p[i]=i;
    }
    int find(int x){
        if(p[x]==x)return x;
        else return p[x]=find(p[x]);
    }
    int query(int x,int y){
        int a=find(x);int b=
 
find(y);
        if(a!=b)p[a]=b;
        if(a==b)return 1;
        return 0;                                  
    }
}S;                                         
void add_edge(int u,int v){
    nex[++cnt]=head[u],to[cnt]=v,head[u]=cnt;
}
void dfs(int u,int fa,int c){
	G[u]=val[u],F[u]=0,vis[u]=1;
	for
 
(int v=head[u];v;v=nex[v])
		if(to[v]!=fa){
			dfs(to[v],u,c);
			G[u]+=F[to[v]];
			F[u]+=max(F[to[v]],G[to[v]]);
		}
}
int main(){
    ll ans=0;
    S.init();
    int n;
    scanf("%d",&n);
    for(int i=1;i<=n;++i){
        int a,b;scanf("%d%d",&a,&b);
        val[i]=a;
        int flag=S.query(i,b);
        if(flag==0){
            add_edge(i,b);
            add_edge(b,i);
        }
        if(flag==1)A[++num]=b,B[num]=i;
    }
    for(int i=1;i<=num;++i){
        int a=A[i],b=B[i];
        ll sum1,sum2;
        root=b;dfs(b,-1,a);sum1=F[b];
        root=a;dfs(a,-1,b);sum2=F[a];
        ans+=max(sum1,sum2);
    }
    for(int i=1;i<=n;++i)
    	if(!vis[i]){
    		dfs(i,-1,-2);
    		ans+=max(F[i],G[i]);
    	}
    printf("%lld",ans);
    return 0;
}