#include<iostream>
#include<sstream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<string>
#include<vector>
#include<cmath>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<numeric>
#include<functional>
#include<complex>

using namespace std;
#define BET(a,b,c) ((a)<=(b)&&(b)<(c))
#define FOR(i,n) for(int i=0,i##_end=(int(n));i<i##_end;i++)
#define SZ(x) (int)(x.size())
#define ALL(x) (x).begin(),(x).end()
#define MP make_pair
#define FOR_EACH(it,v) for(__typeof(v.begin()) it=v.begin(),it_end=v.end() ; it != it_end ; it++)
typedef vector<int> VI;
typedef vector<VI> VVI;

struct StrongConnectComponent
{
    int n ;
    vector<bool> isvisited;
    vector<int> label; 
    vector<vector<int> > adj , inv_adj , scc;
    StrongConnectComponent(vector<vector<int> > &adj) : adj(adj)
    {
        n = (int)adj.size();
        isvisited = vector<bool>(n);
        inv_adj = vector<vector<int> >(n);
        FOR(i,n){
            FOR(j,SZ(adj[i])){
                inv_adj[adj[i][j]].push_back(i);
            }
        }
    }

    void dfs(int k , const vector<vector<int> > &g , vector<int>& lbl)
    {
        if(isvisited[k]) return;
        isvisited[k]=true;
        FOR(i,SZ(g[k])) dfs(g[k][i] , g , lbl) ;
        lbl.push_back(k);
    }

    vector<vector<int> > get_scc()
    {
        fill_n(isvisited.begin() , n , false);
        FOR(i,n) dfs(i , adj , label);
        reverse(label.begin() , label.end()) ;
        fill_n(isvisited.begin() , n , false);
        scc.clear();
        //FOR(i,n) cout<<" "<<label[i]; cout<<endl;
        FOR(i,n) if(!isvisited[label[i]]){
            scc.push_back(vector<int>());
            //cout<<"! "<<label[i]<<endl;
            dfs(label[i], inv_adj , scc.back()) ; 
        }
        return scc; 
    }
};

int main()
{
    int n;
    cin>>n;
    VVI adj(n);
    VI L(n);
    FOR(i,n){
        int l,s;
        cin>>l>>s;
        s--;
        L[i] = l;
        adj[s].push_back(i);
    }
    auto SCC = StrongConnectComponent(adj);
    VVI groups = SCC.get_scc();
    map<int,int> toGroupIndex;
    FOR(i,SZ(groups)) FOR(j,SZ(groups[i])) toGroupIndex[groups[i][j]] = i;
    VI indeg(SZ(groups));
    FOR(i,n) FOR(j,SZ(adj[i])){
        int g1 = toGroupIndex[i];
        int g2 = toGroupIndex[adj[i][j]];
        if(g1 == g2) continue;
        indeg[g2]++;
    }
    double ans = 0 ;
    FOR(i,SZ(groups)){
        double minv = 1e99, sum = 0;
        for(auto p : groups[i]){
            sum += L[p];
            minv = min<double>(minv, L[p]);
        }
        if(indeg[i]){
            ans += sum / 2.0;
        }else{
            ans += (sum - minv) / 2.0 + minv;
        }
    }
    printf("%.1f\n", ans + 1e-12);
    return 0;
}