#include <iostream>
#include <string>
#include <utility>
#include <algorithm>
#include <map>
#include <set>
#include <vector>
#include <cmath>
#include <cstdlib>
#include <queue>
#include <stack>
#include <iomanip>

using namespace std;

#define REP(i, n) for(ll i = 0;i < n;i++)
#define REPR(i, n) for(ll i = n;i >= 0;i--)
#define FOR(i, m, n) for(ll i = m;i < n;i++)
#define FORR(i, m, n) for(ll i = m;i >= n;i--)
#define REPO(i, n) for(ll i = 1;i <= n;i++)
#define ll long long
#define INF (ll)1 << 60
#define MINF (-1 * INF)
#define ALL(n) n.begin(),n.end()
#define MOD 1000000007
#define P pair<ll, ll>

struct UnionFind {
	vector<int> data;
	UnionFind(int size) : data(size, -1) { }
	bool unionSet(int x, int y) {
		x = root(x); y = root(y);
		if (x != y) {
			if (data[y] < data[x]) swap(x, y);
			data[x] += data[y]; data[y] = x;
		}
		return x != y;
	}
	bool findSet(int x, int y) {
		return root(x) == root(y);
	}
	int root(int x) {
		return data[x] < 0 ? x : data[x] = root(data[x]);
	}
	int size(int x) {
		return -data[root(x)];
	}
};

UnionFind uni(110);
ll n, in[110], p[110], now;
float ans = 0, hard[110], mi[110], nhard;
vector<ll> g[110];

bool used[110];
void dfs(ll a) {
	used[a] = true;
	REP(i, g[a].size()) {
		if (!used[g[a][i]]) {
			now++;
			nhard += hard[g[a][i]] / 2;
			dfs(g[a][i]);
		}
	}
}

int main() {
	cin >> n;
	REP(i, 110)mi[i] = 1e9;
	REP(i, n) {
		cin >> hard[i] >> p[i];
		p[i]--;
		uni.unionSet(p[i], i);
		g[p[i]].push_back(i);
	}
	REP(i, n) {
		now = 1;
		nhard = hard[i];
		REP(j, 110)used[j] = false;
		dfs(i);
		if (now == uni.size(i)) {
			mi[uni.root(i)] = min(mi[uni.root(i)], nhard);
		}
	}
	REP(i, 110) {
		if (mi[i] < 1e9)ans += mi[i];
	}
	cout << fixed << setprecision(1) << ans << endl;
}