import std.stdio, std.conv, std.string, std.bigint;
import std.math, std.random, std.datetime;
import std.array, std.range, std.algorithm, std.container, std.format;
string read(){ static string[] ss; while(!ss.length) ss = readln.chomp.split; string res = ss[0]; ss.popFront; return res; }

class Node{
	int id;
	real value;
	Node trigger;
	int count;
	bool isFull = 0;
	bool isVisited = 0;
	this(int id){
		this.id = id;
	}
	override string toString(){
		return "#" ~ id.to!string ~ " => #" ~ (trigger? trigger.id.to!string: "*") ~ " " ~ count.to!string ~ " (" ~ value.format!"%0.1f" ~ ")" ~ (isVisited? (isFull? "": " H"): " ?");
	}
}

void main(){
	int n = read.to!int;
	Node[] nodes;
	foreach(i; 0 .. n) nodes ~= new Node(i);
	foreach(i; 0 .. n){
		Node nd = nodes[i];
		nd.value = read.to!real;
		nd.trigger = nodes[read.to!int - 1];
		nd.trigger.count += 1;
	}
	debug foreach(nd; nodes) nd.writeln;
	
	// 盲腸部分は半減を適用できる
	foreach(nd; nodes) if(! nd.isVisited) for(Node nd1 = nd; nd1.count == 0; nd1 = nd1.trigger){
		nd1.isVisited = 1;
		nd1.trigger.count -= 1;
	}
	debug foreach(nd; nodes) nd.writeln;
	
	// ループ部分(複数の場合もあり)は1つを除き半減を適用できる
	foreach(nd; nodes) if(! nd.isVisited){
		Node bestnd = nd;
		for(Node nd1 = nd; ! nd1.isVisited; nd1 = nd1.trigger){
			nd1.isVisited = 1;
			if(bestnd.value > nd1.value) bestnd = nd1;
		}
		bestnd.isFull= 1;
	}
	debug foreach(nd; nodes) nd.writeln;
	
	// 集計
	real sum = 0.0;
	foreach(nd; nodes) if(nd.isFull) sum += nd.value; else sum += nd.value / 2.0;
	
	sum.format!"%0.1f".writeln;
	
}