#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < n; ++i)

int main(void) {
	int N;
	cin >> N;
	vector<int> L(N), S(N);
	rep(i,N) {
		cin >> L[i] >> S[i];
		--S[i];
	}
	
	vector<int> d(N, 0);
	rep(i,N) ++d[S[i]];
	queue<int> que;
	rep(i,N) if(d[i] == 0) que.push(i);
	while(!que.empty()) {
		int v = que.front();
		que.pop();
		--d[S[v]];
		if(d[S[v]] == 0) que.push(S[v]);
	}
	
	int ans = 0;
	rep(i,N) ans += L[i];
	rep(i,N) {
		if(d[i] == 0) continue;
		int cnt = 1e9, now = i;
		while(d[now] != 0) {
			d[now] = 0;
			cnt = min(cnt, L[now]);
			now = S[now];
		}
		ans += cnt;
	}
	
	cout << ans / 2 << (ans % 2 == 0 ? ".0" : ".5") << endl;
	
	return 0;
}