#include <cstdio>
#include <climits>
#include <cmath>
#include <iostream>
#include <iomanip>
#include <string>
#include <cstdio>
#include <climits>
#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <utility>
#include <queue>
#include <cstring>
#include <set>
#define rep(i, n) for (int i = 0; i < int(n); i++)
using namespace std;
long long MOD = 1000000007;
long long INF = 1000000000000000; //10^15
typedef long long ll;
typedef unsigned long long ull;

ll powMod(ll x, ll n) {
  if (n == 0) return 1;
  ll r = powMod(x, n / 2);
  if (n % 2 == 0) return (r * r) % MOD;
  return (r * r % MOD) * x % MOD;
}

int n, s;
double l[110];
vector<int> g[110];
bool visited[110], minflag;
double sum, Min;

void dfs(int i, double mi, int start) {
  visited[i] = true;
  sum += l[i];
  rep(j, g[i].size()) {
    if (g[i][j] == start) {
      minflag = true;
      Min = mi;
    }
    if (!visited[g[i][j]]) {
      dfs(g[i][j], min(mi, l[g[i][j]]), start);
    }
  }
}


int main(void) {

  cin >> n;

  vector<int> list;
  
  rep(i, n) {
    cin >> l[i] >> s;
    s--;
    if (i == s) list.push_back(s);
    else g[s].push_back(i);
  }

  double ans = 0;

  rep(i, list.size()) {
    sum = 0;
    dfs(list[i], l[list[i]], list[i]);
    ans += sum / 2 + l[list[i]] / 2;
  }

  rep(i, n) {
    minflag = false;
    sum = 0;
    if (!visited[i]) {
      bool vi[110] = {};
      rep(i, n) vi[i] = visited[i];
      dfs(i, l[i], i);
      if (minflag) ans += sum / 2 + Min / 2;
      else {
	rep(i, n) visited[i] = vi[i];
      }
    }
  }

  cout << std::fixed;
  cout << std::setprecision(1) << ans << endl;

  return 0;
}