/* -*- coding: utf-8 -*-
 *
 * 19.cc: No.19 ステージの選択 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100;
const int INF = 1 << 30;

/* typedef */

typedef vector<int> vi;

/* global variables */

int ls[MAX_N];
vi nbrs[MAX_N];
bool used[MAX_N], loop[MAX_N];

/* subroutines */

int rec(int u, int minl = INF) {
  used[u] = loop[u] = true;
  if (minl > ls[u]) minl = ls[u];

  int sum = ls[u];
  vi &nbru = nbrs[u];
  for (vi::iterator vit = nbru.begin(); vit != nbru.end(); vit++) {
    int &v = *vit;
    if (! used[v]) sum += rec(v, minl);
    else if (loop[v]) sum += minl;
  }

  loop[u] = false;
  return sum;
}

/* main */

int main() {
  int n;
  cin >> n;

  for (int i = 0; i < n; i++) {
    int x;
    cin >> ls[i] >> x;
    x--;
    nbrs[x].push_back(i);
  }

  int sum = 0;
  for (int i = 0; i < n; i++)
    if (! used[i]) sum += rec(i);

  printf("%.1lf\n", 0.5 * sum);
  return 0;
}