#include <bits/stdc++.h>
using namespace std;

int main(){
    int n;
    cin>>n;
    vector<int> L(n),S(n);
    for(int i=0;i<n;i++){
        cin>>L[i];
        cin>>S[i];
    }
    vector<bool> cleared(n);
    int pointer;
    int sum,min;
    double ans=0;
    for(int i=0;i<n;i++){
        if(!cleared[i]){
            pointer=i;
            sum=0;
            min=INT_MAX;
            while(!cleared[pointer]){
                if(L[pointer]<min){
                    min=L[pointer];
                }
                sum+=L[pointer];
                cleared[pointer]=true;
                pointer=S[pointer]-1;
            }
            ans+=min+sum;
        }
    }
    printf("%0.1f",ans/2);
}