/*
  少数第一位までだった・・・
 */
#include <bits/stdc++.h>
#define EPS 1e-9
#define INF 1070000000LL
#define MOD 1000000007LL
#define fir first
#define foreach(it,X) for(auto it=(X).begin();it!=(X).end();it++)
#define numa(x,a) for(auto x: a)
#define ite iterator
#define mp make_pair
#define mt make_tuple
#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(int i=m;i<(n);i++)
#define pb push_back
#define pf push_front
#define sec second
#define sz(x) ((int)(x).size())
#define ALL( c ) (c).begin(), (c).end()
#define gcd(a,b) __gcd(a,b)
#define mem(x,n) memset(x,n,sizeof(x))
#define endl "\n"
using namespace std;
template <int POS, class TUPLE> void deploy(std::ostream &os, const TUPLE &tuple){}
template <int POS, class TUPLE, class H, class ...Ts> void deploy(std::ostream &os, const TUPLE &t){ os << (POS == 0 ? "" : ", ") << get<POS>(t); deploy<POS + 1, TUPLE, Ts...>(os, t); }
template <class T,class U> std::ostream& operator<<(std::ostream &os, std::pair<T,U> &p){ os << "(" << p.first <<", " << p.second <<")";return os; }
template <class T> std::ostream& operator<<(std::ostream &os, std::vector<T> &v){ int remain = v.size(); os << "{"; for(auto e: v) os << e << (--remain == 0 ? "}" : ", "); return os; }
template <class T> std::ostream& operator<<(std::ostream &os, std::set<T> &v){ int remain = v.size(); os << "{"; for(auto e: v) os << e << (--remain == 0 ? "}" : ", "); return os; }
template <class T, class K> std::ostream& operator<<(std::ostream &os, std::map<T, K> &mp){ int remain = mp.size(); os << "{"; for(auto e: mp) os << "(" << e.first << " -> " << e.second << ")" << (--remain == 0 ? "}" : ", "); return os; }
#define DEBUG1(var0) { std::cerr << (#var0) << "=" << (var0) << endl; }
#define DEBUG2(var0, var1) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG1(var1); }
#define DEBUG3(var0, var1, var2) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG2(var1,var2); }
#define DEBUG4(var0, var1, var2, var3) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG3(var1,var2,var3); }
using ll = long long;

int N;
vector <int> edge[110];
double difficult[110];

//Union-find木
#define MAX_N 110
int par[MAX_N];
int myrank[MAX_N];
int team[MAX_N];
double team_cost[MAX_N];
void init(int n){//要素初期化
  rep(i,n){
    par[i] = i;
    myrank[i] = 0;
  }
}

int find(int x){//木の根を求める
  if(par[x] == x){
    return x;
  }else{
    return par[x] = find(par[x]);
  }
}

void unite(int x,int y){//xとyの属する集合を併合
  x = find(x);
  y = find(y);
  if(x == y){
    return;
  }
  if(myrank[x] < myrank[y]){
    par[x] = y;
  }else{
    par[y] = x;
    if(myrank[x] == myrank[y]){
      myrank[x] += 1;
    }
  }
}

bool same(int x,int y){//x,yが同じ集合に属するか否か
  return find(x) == find(y);
}


double ans = 0;
bool used[110];
void dfs(int now,int par = -1){
  if(used[now]){
    return;
  }
  used[now] = true;
  if(par != -1){
    ans += difficult[now]/2;
  }else{
    ans += difficult[now];
  }
  numa(x,edge[now]){
    dfs(x,now);
  }
}

int count_dfs(int now){
  if(used[now]){
    return 0;
  }
  used[now] = true;
  int ret = 1;
  numa(x,edge[now]){
    ret += count_dfs(x);
  }
  return ret;
}
int main()
{
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cin >> N;
  init(N+5);
  rep2(i,1,N+1){
    int l,s;
    cin >> l >> s;
    difficult[i] = l;
    unite(i,s);
    edge[s].pb(i);
  }
  rep2(i,1,N+1){
    team[find(i)] += 1;
    team_cost[find(i)] = INF;
  }

  rep2(i,1,N+1){
    mem(used,0);
    auto node_cnt = count_dfs(i);
    if(node_cnt == team[find(i)]){
      mem(used,0);
      ans = 0;
      dfs(i);
      team_cost[find(i)] = min(team_cost[find(i)],ans);
    }
  }
  double final_ans = 0;
  rep2(i,1,N+1){
    final_ans += team_cost[i];
  }
  cout << fixed << setprecision(1) << final_ans << endl;
  return 0;
}