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

pair<vector<int>,vector<vector<int>>> FunctionalGraph(vector<int> &A){
    //N頂点N辺有向出次数全て1のグラフでreturn{cycle所属番号(-1は未所属),cycle頂点集合}.
    int N = A.size(),nowc = 0;
    vector<int> belong(N,-1);
    vector<vector<int>> rev(N),cycle;
    for(int i=0; i<N; i++) rev.at(A.at(i)).push_back(i);
    vector<bool> already(N),visited(N);
    for(int i=0; i<N; i++){
        if(already.at(i)) continue;
        int pos = i;
        while(!visited.at(pos)) visited.at(pos) = true,pos = A.at(pos);
        int memo = pos;
        cycle.push_back({pos}); belong.at(pos) = nowc; pos = A.at(pos);
        while(pos != memo){
            cycle.at(nowc).push_back(pos); belong.at(pos) = nowc;
            pos = A.at(pos);
        }
        already.at(pos) = true;
        queue<int> Q; Q.push(pos);
        while(Q.size()){
            int pos2 = Q.front(); Q.pop();
            for(auto to : rev.at(pos2)){
                if(already.at(to)) continue;
                already.at(to) = true; Q.push(to);
            }
        }
        nowc++;
    }
    return {belong,cycle};
}

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int N; cin >> N;
    vector<int> L(N),A(N);
    for(int i=0; i<N; i++) cin >> L.at(i) >> A.at(i),A.at(i)--;
    
    auto [belong,cycle] = FunctionalGraph(A);
    int answer = 0;
    for(int i=0; i<N; i++) if(belong.at(i) == -1) answer += L.at(i);
    for(auto c : cycle){
        int sum = 0,mins = 1001001001;
        for(auto pos : c) sum += L.at(pos),mins = min(mins,L.at(pos));
        answer += sum+mins;
    }
    cout << answer/2 << "." << (answer%2?5:0) << endl;

}