#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <cstring>

typedef long long ll;
using namespace std;

#define mod 1000003
#define INF 1000000000
#define LLINF 2000000000000000000LL

#define SIZE 10000

int n,L[100],S;
vector<int> way[100],wayback[100],group[101];
int visit[100],cc=0,group_in[100];
int dfsnum[100],dfsnumdic[100],groupnum[100];
bool scckaku[100];
queue<int> que;


void sccdfs(int now){
    
    if(visit[now]) return;
    visit[now]=true;
    
    dfsnum[now]=cc;
    dfsnumdic[cc]=now;
    cc++;
    
    cerr << cc << endl;
    
    for(int i=0;i<way[now].size();i++){
        sccdfs(way[now][i]);
    }
    
    return;
}

void make_group(int now,int groupN){
    
    if(visit[now]) return;
    visit[now]=true;
    
    group[groupN].push_back(now);
    groupnum[now] = groupN;

    for(int i=0;i<wayback[now].size();i++){
        make_group(wayback[now][i], groupN);
    }
    
}

void scc(){
    int num = 0;
    
    for(int i=0;i<n;i++){
        sccdfs(i);
    }
    
    for(int i=0;i<n;i++)
        visit[i]=false;
    
    for(int i=n-1;i>=0;i--){
        if(!visit[dfsnumdic[i]]){
            make_group(dfsnumdic[i],num);
            num++;
        }
    }
    
    for(int i=0;i<n;i++){
        for(int j=0;j<way[i].size();j++){
            if(groupnum[i]!=groupnum[way[i][j]]){
                group_in[groupnum[way[i][j]]]++;
            }
        }
    }
}

int main(){
    double ans;
    
    scanf("%d",&n);
    
    for(int i=0;i<n;i++){
        scanf("%d%d",&L[i],&S);
        S--;
        
        if(i!=S){
            way[S].push_back(i);
            wayback[i].push_back(S);
        }
    }
    
    scc();
    
    for(int i=0;i<n;i++){
        int min_l=INF;
        
        if(group[i].empty()) continue;
        
        for(int j=0;j<group[i].size();j++){
            ans+=L[group[i][j]]/2.0;
            min_l = min(min_l,L[group[i][j]]);
        }
        
        if(group_in[i]==0)
            ans+=min_l/2.0;
    }
    
    printf("%.1lf\n",ans);
    
    return 0;
}