#include "bits/stdc++.h"
using namespace std;

bool dist[100][100];
int main() {
	int N;
	cin >> N;
	vector<int> L(N), S(N);

	int ans = 0;
	for (int i = 0; i < N; i++)
	{
		cin >> L[i] >> S[i];
		S[i]--;
		dist[i][S[i]] = true;
		dist[i][i] = true;
		ans += L[i];
	}

	for (int k = 0; k < N; k++)
	{
		for (int i = 0; i < N; i++)
		{
			for (int j = 0; j < N; j++)
			{
				dist[i][j] |= dist[i][k] & dist[k][j];
			}
		}
	}

	for (int i = 0; i < N; i++)
	{
		bool flag = true;
		for (int j = 0; j < i; j++)
		{
			if (dist[i][j] && dist[j][i]) flag = false;
		}
		if (!flag) continue;
		for (int j = 0; j < N; j++)
		{
			if (dist[i][j] && !dist[j][i]) flag = false;
		}
		if (!flag) continue;
		int mincost = 999;
		for (int j = i; j < N; j++)
		{
			if (dist[i][j] && dist[j][i]) mincost = min(mincost, L[j]);
		}
		ans += mincost;
	}
	printf("%.1f\n", ans / 2.0);
}