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 G = Enu.Range(0, N).Select(i => new List<int>()).ToArray();

        for (int i = 0; i < N; i++)
        {
            L[i] = reader.Int() * 2;
            S[i] = reader.Int() - 1;
            G[S[i]].Add(i);
        }
        var scc = new SCC(G);
        var numCompo = Enu.Range(0, N).Max(i => scc[i]);
        int ans = 0;
        bool[] seen = new bool[N];
        for (int c = 0; c <= numCompo; c++)
        {
            var vs = Enu.Range(0, N).Where(i => scc[i] == c).ToList();
            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;
            }
            vs.ForEach(v => seen[v] = true);
        }
        Console.WriteLine((ans / 2.0).ToString("f1"));
    }


    class SCC
    {
        private readonly int[] component;
        public int this[int v] { get { return component[v]; } }

        public SCC(List<int>[] G)
        {
            var seen = new bool[G.Length];
            var vs = new List<int>();
            var RevE = Enu.Range(0, G.Length).Select(i => new List<int>()).ToArray();
            for (int a = 0; a < G.Length; a++)
                foreach (int b in G[a])
                    RevE[b].Add(a);
            component = Enu.Repeat(-1, G.Length).ToArray();
            int num = 0;
            for (int v = 0; v < G.Length; v++)
                if (!seen[v]) DFS(v, seen, G, vs);
            vs.Reverse();
            foreach (int v in vs)
                if (component[v] == -1) DFSRev(v, num++, RevE);
        }
        void DFS(int v, bool[] seen, List<int>[] G, List<int> vs)
        {
            seen[v] = true;
            foreach (int next in G[v])
                if (!seen[next]) DFS(next, seen, G, vs);
            vs.Add(v);
        }
        void DFSRev(int v, int g, List<int>[] RevE)
        {
            component[v] = g;
            foreach (int next in RevE[v])
                if (component[next] == -1) DFSRev(next, g, RevE);
        }
    }



    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++]; }
}