using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Diagnostics;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using Enu = System.Linq.Enumerable;

class Program
{
    static readonly int INF = (int)1e6;

    void Solve()
    {
        int N = reader.Int();
        int[] L = new int[N], S = new int[N];
        var scc = new SCC(N);

        for (int i = 0; i < N; i++)
        {
            L[i] = reader.Int() * 2;
            S[i] = reader.Int() - 1;
            scc.AddEdge(S[i], i);
        }
        scc.Decompose();
        int ans = 0;
        bool[] seen = new bool[N];
        for (int c = 0; c < scc.NumComponents; c++)
        {
            var vs = scc.Vertexes(c);
            int sum = vs.Sum(v => L[v]);
            if (vs.Any(v => seen[S[v]])) ans += sum / 2;
            else
            {
                int min = vs.Min(v => L[v]);
                ans += (sum - min) / 2 + min;
            }
            Array.ForEach(vs, v => seen[v] = true);
        }
        Console.WriteLine((ans / 2.0).ToString("f1"));
    }


    class SCC
    {
        public int NumComponents { get; private set; }
        public int[] Vertexes(int at) { return vertexes[at].ToArray(); }
        public int Component(int v) { return component[v]; }
        private readonly bool[] seen;
        private readonly int[] component;
        private readonly List<int> vs = new List<int>();
        private readonly List<int>[] E, RevE;
        private readonly List<List<int>> vertexes = new List<List<int>>();

        public SCC(int N)
        {
            vs = new List<int>();
            seen = new bool[N];
            component = new int[N];
            E = new List<int>[N];
            RevE = new List<int>[N];
            for (int i = 0; i < N; i++)
            {
                component[i] = -1;
                E[i] = new List<int>();
                RevE[i] = new List<int>();
            }
        }
        public void AddEdge(int from, int to)
        {
            E[from].Add(to);
            RevE[to].Add(from);
        }
        public void Decompose()
        {
            for (int v = 0; v < E.Length; v++)
                if (!seen[v]) DFS(v);
            vs.Reverse();
            foreach (int v in vs)
                if (component[v] == -1)
                {
                    vertexes.Add(new List<int>());
                    DFSRev(v, NumComponents++);
                }
        }
        private void DFS(int v)
        {
            seen[v] = true;
            foreach (int next in E[v])
                if (!seen[next]) DFS(next);
            vs.Add(v);
        }
        private void DFSRev(int v, int num)
        {
            component[v] = num;
            vertexes[num].Add(v);
            foreach (int next in RevE[v])
                if (component[next] == -1) DFSRev(next, num);
        }
    }



    Reader reader = new Reader(Console.In);
    static void Main() { new Program().Solve(); }
}

class Reader
{
    private readonly TextReader reader;
    private readonly char[] separator = { ' ' };
    private readonly StringSplitOptions removeOp = StringSplitOptions.RemoveEmptyEntries;
    private string[] A = new string[0];
    private int i;

    public Reader(TextReader r) { reader = r; }
    public bool HasNext() { return Enqueue(); }
    public string String() { return Dequeue(); }
    public int Int() { return int.Parse(Dequeue()); }
    public long Long() { return long.Parse(Dequeue()); }
    public double Double() { return double.Parse(Dequeue()); }
    public int[] IntLine() { var s = Line(); return s == "" ? new int[0] : Array.ConvertAll(Split(s), int.Parse); }
    public int[] IntArray(int N) { return Enu.Range(0, N).Select(i => Int()).ToArray(); }
    public int[][] IntGrid(int H) { return Enu.Range(0, H).Select(i => IntLine()).ToArray(); }
    public string[] StringArray(int N) { return Enu.Range(0, N).Select(i => Line()).ToArray(); }
    public string Line() { return reader.ReadLine().Trim(); }
    private string[] Split(string s) { return s.Split(separator, removeOp); }
    private bool Enqueue()
    {
        if (i < A.Length) return true;
        string line = reader.ReadLine();
        if (line == null) return false;
        if (line == "") return Enqueue();
        A = Split(line);
        i = 0;
        return true;
    }
    private string Dequeue() { Enqueue(); return A[i++]; }
}