#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstring>
#include <cmath>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<int, ll> i_ll;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int u, v; ll w; };

ll MOD = 1000000007;
ll _MOD = 1000000009;
double EPS = 1e-10;

int main() {
	int N; cin >> N;
	vector<int> L(N), S(N);
	for (int i = 0; i < N; i++)
		cin >> L[i] >> S[i];
	int sum = 0;
	for (int i = 0; i < N; i++)
		sum += L[i];
	vector<bool> vis(N);
	for (int i = 0; i < N; i++) {
		if (vis[i]) continue;
		bool loop = false;
		int j = i;
		for (int t = 0; t < N; t++) {
			j = S[j] - 1;
			if (j == i) {
				loop = true;
				break;
			}
		}
		if (!loop) continue;
		int mini = INT_MAX;
		for (int j = i; !vis[j]; j = S[j] - 1) {
			vis[j] = true;
			mini = min(mini, L[j]);
		}
		sum += mini;
	}
	printf("%.1f\n", sum / 2.0);
}