#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>
#include <set>
using namespace std;

class UnionFindTree{
	typedef struct {
		int parent;
		int rank;
	}base_node;
	
	vector<base_node> node;
public:
	UnionFindTree(int n){
		node.resize(n);
		for(int i=0; i<n; i++){
			node[i].parent=i;
			node[i].rank=0;
		}
	}

	int find(int x){
		if(node[x].parent == x) return x;
		else{
			return node[x].parent = find(node[x].parent);
		}
	}
	
	bool same(int x, int y){
		return find(x) == find(y);
	}

	void unite(int x, int y){
		x = find(node[x].parent);
		y = find(node[y].parent);
		if(x==y) return;
		if(node[x].rank < node[y].rank){
			node[x].parent = y;
		}else if(node[x].rank > node[y].rank){
			node[y].parent = x;
		}else{
			node[x].rank++;
			unite(x,y);
		}
	}
};


int main(){
	int n;
	cin >> n;
	vector<long long> L(n), S(n);

	UnionFindTree uft(n);
	long long sum = 0;
	for(int i=0; i<n; i++){
		cin >> L[i] >> S[i];
		S[i]--;
		sum += L[i];

		uft.unite(i, S[i]);
	}

	long long hoge = 0;

	vector<bool> visit(n, false);
	for(int i=0; i<n; i++){
		if(visit[uft.find(i)]) continue;

		int pos = i;
		while(visit[pos] == false){
			visit[pos] = true;
			pos = S[pos];
		}
		int start = pos;
		long long cost = L[pos];
		pos = S[pos];
		while(pos != start){
			cost = min(cost, L[pos]);
			pos = S[pos];
		}
		hoge += cost;

		visit[uft.find(pos)] = true;
	}

	double ans = sum/2.0 + hoge/2.0;
	printf("%.1f\n", ans);


	return 0;
}