#include <bits/stdc++.h>
using namespace std;
const int INF = 100000;
struct strongly_connected_components{
  int cnt;
  vector<int> scc;
  void dfs(vector<vector<int>> &E, vector<bool> &used, vector<int> &ord, int v){
    used[v] = true;
    for (int w : E[v]){
      if (!used[w]){
        dfs(E, used, ord, w);
      }
    }
    ord.push_back(v);
  }
  strongly_connected_components(vector<vector<int>> &E){
    int N = E.size();
    vector<vector<int>> E2(N);
    for (int i = 0; i < N; i++){
      for (int j : E[i]){
        E2[j].push_back(i);
      }
    }
    vector<int> ord;
    vector<bool> used1(N, false);
    for (int i = 0; i < N; i++){
      if (!used1[i]){
        dfs(E2, used1, ord, i);
      }
    }
    reverse(ord.begin(), ord.end());
    cnt = 0;
    scc = vector<int>(N, -1);
    for (int i = 0; i < N; i++){
      if (scc[ord[i]] == -1){
        scc[ord[i]] = cnt;
        queue<int> Q;
        Q.push(ord[i]);
        while (!Q.empty()){
          int v = Q.front();
          Q.pop();
          for (int w : E[v]){
            if (scc[w] == -1){
              scc[w] = cnt;
              Q.push(w);
            }
          }
        }
        cnt++;
      }
    }
  }
  int size(){
    return cnt;
  }
  int operator [](int k){
    return scc[k];
  }
};
int main(){
  cout << fixed << setprecision(20);
  int N;
  cin >> N;
  vector<int> L(N), S(N);
  for (int i = 0; i < N; i++){
    cin >> L[i] >> S[i];
    S[i]--;
  }
  vector<vector<int>> E(N);
  for (int i = 0; i < N; i++){
    E[S[i]].push_back(i);
  }
  strongly_connected_components G(E);
  int M = G.size();
  vector<int> d(M, 0);
  for (int i = 0; i < N; i++){
    if (G[S[i]] != G[i]){
      d[G[i]]++;
    }
  }
  vector<int> mn(M, INF);
  for (int i = 0; i < N; i++){
    mn[G[i]] = min(mn[G[i]], L[i]);
  }
  double ans = 0;
  for (int i = 0; i < N; i++){
    ans += (double) L[i] / 2;
  }
  for (int i = 0; i < M; i++){
    if (d[i] == 0){
      ans += (double) mn[i] / 2;
    }
  }
  cout << ans << endl;
}