#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<ll, ll> p_ll;

template<class T>
void debug(T itr1, T itr2) { auto now = itr1; while(now<itr2) { cout << *now << " "; now++; } cout << endl; }
#define repr(i,from,to) for (int i=(int)from; i<(int)to; i++)
#define all(vec) vec.begin(), vec.end()
#define rep(i,N) repr(i,0,N)
#define per(i,N) for (int i=(int)N-1; i>=0; i--)

const ll MOD = pow(10,9)+7;
const ll LLINF = pow(2,61)-1;
const int INF = pow(2,30)-1;

vector<ll> fac;
void c_fac(int x=pow(10,6)+10) { fac.resize(x,true); rep(i,x) fac[i] = i ? (fac[i-1]*i)%MOD : 1; }
ll inv(ll a, ll m=MOD) { ll b = m, x = 1, y = 0; while (b!=0) { int d = a/b; a -= b*d; swap(a,b); x -= y*d; swap(x,y); } return (x+m)%m; }
ll nck(ll n, ll k) { return fac[n]*inv(fac[k]*fac[n-k]%MOD)%MOD; }
ll gcd(ll a, ll b) { if (a<b) swap(a,b); return b==0 ? a : gcd(b, a%b); }
ll lcm(ll a, ll b) { return a/gcd(a,b)*b; }

vector<double> vtx;
vector<ll> a_s;
vector<vector<ll>> adj, radj;
vector<bool> used;

double result = 0;

void dfs_to(int n) {
  for (auto x: adj[n]) {
    if (used[x]) continue;
    used[x] = true;
    dfs_to(x);
    result += vtx[x]/2;
  }
  // cout << " ~" << n << "/2 -> " << result << endl;
}

void dfs_back(int n) {
  result += radj[n].size() ? vtx[n] / 2 : vtx[n];
  // cout << " ~" << n << "/2 -> " << result << endl;
  for (auto x: radj[n]) {
    if (used[x]) continue;
    a_s[x]--;
    if (a_s[x]==0) {
      used[x] = true;
      dfs_back(x);
    }
  }
}

void dfs_loop(int n, double mi) {
  // cout << n << "-" << mi << endl;
  used[n] = true; result += vtx[n]/2;
  bool ok = false;
  for (auto x: adj[n]) {
    if (used[x]) continue;
    ok = true; mi = min(mi, vtx[x]);
    dfs_loop(x, mi);
  }
  if (!ok) result += mi/2;
}

int main() {
  int N; cin >> N;
  vtx.resize(N); adj.resize(N), radj.resize(N); a_s.resize(N);
  rep(i,N) {
    ll L; double S; cin >> L >> S; S--;
    vtx[i] = L;
    if (i!=S) { 
      adj[S].push_back(i); a_s[S]++;
      radj[i].push_back(S); 
    }
  }

  used.resize(N,false);
  rep(i,N) {
    if (!used[i]&&radj[i].size()==0) {
      // cout << i << endl;
      result += vtx[i];
      used[i] = true;
      dfs_to(i);
    }
  }

  rep(i,N) {
    if (!used[i]&&a_s[i]==0) {
      // cout << i << "/2" << endl;
      used[i] = true; 
      dfs_back(i);
    }
  }

  // debug(all(used));
  // rep(i,N) cout << radj[i].size() << " "; cout << endl;

  rep(i,N) if (!used[i]) dfs_loop(i,vtx[i]);
  printf("%.1f", result); cout << endl;
  return 0;
}