using System;
using System.Collections.Generic;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.IO;
using System.Reflection;
using static System.Math;
using System.Numerics;
static class Program{
	const int mod=(int)1e9+7;
	static List<int>[] li,lir,lid;
	static bool[] b;
	static int k=0,m=0;
	static int[] h,zt;
	static void Main(){
		Sc sc=new Sc();
		var n=sc.I;
		li=new List<int>[n+1];
		lir=new List<int>[n+1];
		b=new bool[n+1];
		k=n-1;
		for(int i=1;i<=n;i++){
			li[i]=new List<int>();
			lir[i]=new List<int>();
		}
		var c=new int[n+1];
		for(int i=1;i<=n;i++){
			int[] e=sc.Ia;
			c[i]=e[0];
			li[e[1]].Add(i);
			lir[i].Add(e[1]);
		}
		h=new int[n];
		for(int i=1;i<=n;i++){if(!b[i]){Fu1(i);}}
		b=new bool[n+1];
		zt=new int[n+1];
		for(int i=0;i<n;i++){
			if(!b[h[i]]){
				m++;
				Fu2(h[i]);
			}
		}
		lid=new List<int>[m+1];
		var d=new int[m+1];
		for(int i=1;i<=m;i++){lid[i]=new List<int>();}
		c[0]=int.MaxValue;
		for(int i=1;i<=n;i++){
			for(int j=0;j<li[i].Count;j++){if(zt[i]!=zt[li[i][j]]){lid[zt[li[i][j]]].Add(zt[i]);}}
		}
		long ans=0;
		for(int i = 1;i<=n;i++) {
			if(c[d[zt[i]]]>c[i]){d[zt[i]]=i;}
		}
		for(int i = 1;i<=n;i++) {
			if(lid[zt[i]].Count!=0||d[zt[i]]!=i){ans+=c[i];}
			else{ans+=c[i]<<1;}
		}
		Console.WriteLine("{0:f1}",ans*1.0/2);
	}
	static void Fu1(int a){
		b[a]=true;
		for(int i=0;i<li[a].Count;i++){if(!b[li[a][i]]){Fu1(li[a][i]);}}
		h[k]=a;
		k--;
	}
	static void Fu2(int a){
		zt[a]=m;
		b[a]=true;
		for(int i=0;i<lir[a].Count;i++){if(!b[lir[a][i]]){Fu2(lir[a][i]);}}
	}
}

public class Sc{
	public int I{get{return int.Parse(Console.ReadLine());}}
	public long L{get{return long.Parse(Console.ReadLine());}}
	public double D{get{return double.Parse(Console.ReadLine());}}
	public string S{get{return Console.ReadLine();}}
	public int[] Ia{get{return Array.ConvertAll(Console.ReadLine().Split(),int.Parse);}}
	public long[] La{get{return Array.ConvertAll(Console.ReadLine().Split(),long.Parse);}}
	public double[] Da{get{return Array.ConvertAll(Console.ReadLine().Split(),double.Parse);}}
	public string[] Sa{get{return Console.ReadLine().Split();}}
	public object[] Oa{get{return Console.ReadLine().Split();}}
	public int[] Ia2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),int.Parse);}}
	public int[] Ia3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),int.Parse);}
	public int[] Ia3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),int.Parse);}
	public long[] La2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),long.Parse);}}
	public long[] La3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),long.Parse);}
	public long[] La3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),long.Parse);}
	public T[] Arr<T>(int n,Func<T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f();}return a;}
	public T[] Arr<T>(int n,Func<int,T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i);}return a;}
	public T[] Arr<T>(int n,Func<string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(Console.ReadLine().Split());}return a;}
	public T[] Arr<T>(int n,Func<int,string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i,Console.ReadLine().Split());}return a;}
}