#include <bits/stdc++.h>
using namespace std;

vector<vector<int>> find_cycle(vector<int> to) {
  int n = to.size();
  vector<vector<int>> ret;
  vector<int> vec(n, -1);
  for (int v = 0; v < n; v++) {
    if (vec.at(v) == -1) {
      int nv = v;
      do {
        vec.at(nv) = v;
        nv = to.at(nv);
      } while (vec.at(nv) == -1);
      if (vec.at(nv) == v) {
        vector<int> tmp;
        int end = nv;
        do {
          tmp.push_back(nv);
          nv = to.at(nv);
        } while (nv != end);
        ret.push_back(tmp);
      }
    }
  }
  return ret;
}

int main() {
  int N;
  cin >> N;
  vector<int> L(N), S(N);
  int sum = 0;
  for (int i = 0; i < N; i++) {
    cin >> L.at(i) >> S.at(i);
    sum += L.at(i);
    S.at(i)--;
  }
  for (auto C : find_cycle(S)) {
    int mi = INT_MAX;
    for (auto c : C) mi = min(mi, L.at(c));
    sum += mi;
  }
  cout << fixed << setprecision(1) << sum / 2.0 << "\n";
}