#include <iostream>
#define REP(i,n) for(int i=0; i<(int)(n); i++)

#include <queue>
#include <cstdio>
inline int getInt(){ int s; scanf("%d", &s); return s; }

#include <set>

using namespace std;

class UnionFind{
private:
  mutable vector<int> id;

  int getId(int i) const{
    if(i == id[i]) return i;
    return id[i] = getId(id[i]);
  }
public:
  UnionFind(int size = 0){ init(size); }
  ~UnionFind(){}
  void init(int size){
    id = vector<int>(size);
    for(int i = 0; i < size; i++)
      id[i] = i;
  }
  void unite(int i, int j){
    int next = min( getId(i), getId(j) );
    id[getId(i)] = id[getId(j)] = next;
  }
  int operator [](int i) const{ return getId(i); }
  int count() const{
    set<int> s;
    for(int i = 0; i < (int)id.size(); i++)
      s.insert(getId(i));
    return s.size();
  }
};


int l[100];
vector<int> s[100];

int f[100];
pair<int, double> dfs(int pos, int d = 0){
  if(f[pos]) return make_pair(0, 0.);

  int cnt = 1;
  double ret = l[pos];
  if(d) ret /= 2;

  f[pos] = 1;
  REP(i,s[pos].size()){
    pair<int, double> tmp = dfs(s[pos][i], 1);
    cnt += tmp.first;
    ret += tmp.second;
  }
  f[pos] = 0;

  return make_pair(cnt, ret);
}

int main(){
  const int n = getInt();
  UnionFind uf(n);

  REP(i,n){
    l[i] = getInt();
    const int ss = getInt() - 1;
    s[ss].push_back(i);
    uf.unite(i, ss);
  }

  vector<int> cnts(n);
  REP(i,n) cnts[uf[i]]++;

  vector<double> memo(n, 1e10);
  REP(i,n){
    pair<int, double> tmp = dfs(i);
    if(tmp.first == cnts[uf[i]])
      memo[uf[i]] = min(memo[uf[i]], tmp.second);
  }

  double ans = 0;
  REP(i,n) if(uf[i] == i) ans += memo[i];

  printf("%.1f\n", ans);

  return 0;
}