using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;
 
class Myon
{
    public Myon() { }
    public static int Main()
    {
        new Myon().calc();
        return 0;
    }
 
    bool[] used;
    int ret;
    int[] L, S;
    List<int>[] rev;
 
    void calc()
    {
        Scanner cin = new Scanner();
        int n = cin.nextInt();
        L = new int[n];
        S = new int[n];
 
        for (int i = 0; i < n; i++)
        {
            L[i] = cin.nextInt();
            S[i] = cin.nextInt() - 1;
        }
 
        int[] innum = new int[n];
        rev = new List<int>[n];
 
        for (int i = 0; i < n; i++)
        {
            rev[i] = new List<int>();
        }
 
        for (int i = 0; i < n; i++)
        {
            innum[S[i]]++;
            rev[S[i]].Add(i);
        }
 
        used = new bool[n];
        ret = 0;
 
        while (true)
        {
            bool flag = false;
            for (int i = 0; i < n; i++)
            {
                if (innum[i] == 0 && !used[i])
                {
                    ret += L[i];
                    used[i] = true;
                    innum[S[i]]--;
                    flag = true;
                }
            }
            if (!flag) break;
        }
 
 
        for (int i = 0; i < n; i++)
        {
            if (!used[i])
            {
                int temp = dfs(i);
                ret += temp;
            }
        }
        Console.WriteLine("{0:0.0}", ret / 2.0);
    }
 
    int dfs(int a)
    {
        if (used[a]) return 99999999;
        used[a] = true;
 
        int r = L[a];
        ret += L[a];
 
        foreach (var item in rev[a])
        {
            r = Math.Min(dfs(item), r);
        }
 
        return r;
    }
 
 
}
 
 
 
 
class Scanner
{
    string[] s;
    int i;
 
    char[] cs = new char[] { ' ' };
 
    public Scanner()
    {
        s = new string[0];
        i = 0;
    }
 
    public string next()
    {
        if (i < s.Length) return s[i++];
        do
        {
            s = Console.ReadLine().Split(cs, StringSplitOptions.RemoveEmptyEntries);
        } while ((s.Length == 1 && s[0] == "") || s.Length == 0);
        i = 0;
        return s[i++];
    }
 
    public int nextInt()
    {
        return int.Parse(next());
    }
 
    public long nextLong()
    {
        return long.Parse(next());
    }
 
    public double nextDouble()
    {
        return double.Parse(next());
    }
 
}
 

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85 19
11 20
65 55
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70 99
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15 67
80 19
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