import std.algorithm, std.conv, std.range, std.stdio, std.string;

void main()
{
  auto n = readln.chomp.to!size_t;

  auto li = new int[](n);
  auto uf = new UnionFind!size_t(n);
  auto pi = new size_t[](n);
  pi[] = n;

  foreach (i; n.iota) {
    auto rd = readln.split, l = rd[0].to!int, s = rd[1].to!size_t - 1;
    li[i] = l;
    uf.unite(s, i);
    pi[i] = s;
  }

  size_t[][size_t] ci;
  foreach (i; n.iota) ci[uf.find(i)] ~= i;

  auto findRoot(size_t[] c) {
    auto vi = new bool[](n);
    auto d = c.front;
    vi[d] = true;
    while (!pi[d] < n) {
      if (vi[pi[d]]) {
        auto lp = [pi[d]];
        vi[] = false;
        while (!vi[lp.back]) {
          lp ~= pi[lp.back];
          vi[lp.back] = true;
        }
        return lp ~ pi[lp.back];
      }
      d = pi[d];
      vi[d] = true;
    }
    return [d];
  }

  auto pt = 0;
  foreach (c; ci.byValue) {
    auto ri = findRoot(c);
    pt += ri.map!(r => li.indexed(c).sum + li[r]).fold!min;
  }

  writefln("%.1f", pt.to!real / 2);
}

class UnionFind(T) {
  T[] p; // parent
  T s; // sentinel

  this(T n) {
    p = new T[](n);
    s = n + 1;
    p[] = s;
  }

  T find(T i) {
    if (p[i] == s) {
      return i;
    } else {
      p[i] = find(p[i]);
      return p[i];
    }
  }

  void unite(T i, T j) {
    auto pi = find(i), pj = find(j);
    if (pi != pj) p[pj] = pi;
  }
}