#include<bits/stdc++.h>
using namespace std;
#define inf INT_MAX
#define INF LLONG_MAX
#define ll long long
#define ull unsigned long long
#define M (int)(1e9+7)
#define P pair<int,int>
#define PLL pair<ll,ll>
#define FOR(i,m,n) for(int i=(int)m;i<(int)n;i++)
#define RFOR(i,m,n) for(int i=(int)m;i>=(int)n;i--)
#define rep(i,n) FOR(i,0,n)
#define rrep(i,n) RFOR(i,n,0)
#define all(a) a.begin(),a.end()
#define IN(a,n) rep(i,n){ cin>>a[i]; }
const int vx[4] = {0,1,0,-1};
const int vy[4] = {1,0,-1,0};
#define PI 3.14159265
#define F first
#define S second
#define PB push_back
#define EB emplace_back
#define int ll
#define vi vector<int>
#define PP pair<int,P>



template< typename T >
struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}

  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};

template< typename T >
using Edges = vector< edge< T > >;
template< typename T >
using WG = vector< Edges< T > >;
using UG = vector< vector< int > >;
template< typename T >
using Matrix = vector< vector< T > >;



template< typename G >
struct StronglyConnectedComponents {
  const G &g;
  UG gg, rg;
  vector< int > comp, order, used;

  StronglyConnectedComponents(G &g) : g(g), gg(g.size()), rg(g.size()), comp(g.size(), -1), used(g.size()) {
    for(int i = 0; i < g.size(); i++) {
      for(auto e : g[i]) {
        gg[i].emplace_back((int) e);
        rg[(int) e].emplace_back(i);
      }
    }
  }

  int operator[](int k) {
    return comp[k];
  }

  void dfs(int idx) {
    if(used[idx]) return;
    used[idx] = true;
    for(int to : gg[idx]) dfs(to);
    order.push_back(idx);
  }

  void rdfs(int idx, int cnt) {
    if(comp[idx] != -1) return;
    comp[idx] = cnt;
    for(int to : rg[idx]) rdfs(to, cnt);
  }

  void build(UG &t) {
    for(int i = 0; i < gg.size(); i++) dfs(i);
    reverse(begin(order), end(order));
    int ptr = 0;
    for(int i : order) if(comp[i] == -1) rdfs(i, ptr), ptr++;

    t.resize(ptr);
    for(int i = 0; i < g.size(); i++) {
      for(auto &to : g[i]) {
        int x = comp[i], y = comp[to];
        if(x == y) continue;
        t[x].push_back(y);
      }
    }
  }
};

signed main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int n;
  cin>>n;
  UG es(n),buff;
  vi v(n);
  rep(i,n){
    int a;
    cin>>v[i]>>a;
    a--;
    es[a].PB(i);
  }

  StronglyConnectedComponents<UG> scc(es);
  scc.build(buff);

  vector<PP> p(n);
  rep(i,n){
    //cout<<scc[i]<<endl;
    p[i]=PP(scc[i],P(v[i],i));
  }

  sort(all(p));
  vector<bool> b(n+1,false);
  

  double ans=0;
  rep(i,n){
    int x=p[i].F;
    double k=p[i].S.F;
    if(b[x]) k/=2;
    b[x]=true;
    ans+=k;
    //cout<<k<<' ';
    for(auto e:es[p[i].S.S]){
      b[scc[e]]=true;
    }
  }
  //cout<<endl;

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

}