#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef unsigned long long ULL;

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(auto i: c)
#define SORT(c) sort((c).begin(),(c).end())
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()

#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	int N;
	cin >> N;
	VI L(N), S(N, -1), done(N, false);
	VVI C(N);

	double ret = 0;

	REP(i, N){
		int s;
		cin >> L[i] >> s;
		s--;

		if(i == s){
			ret += L[i];
			done[i] = true;
			continue;
		}

		S[i] = s;
		C[s].push_back(i);
	}

	auto checkLoop = [&](int i){
		if(done[i]) return;

		int cur = i;
		VI d(N, false);
		bool loop = false;

		while(S[cur] != -1 || !done[cur]){
			if(d[cur]){
				loop = true;
				break;
			}
			d[cur] = true;
			cur = S[cur];
		}

		if(loop){
			d = VI(N, false);
			int mi = 100;
			while(d[cur] == false){
				ret += L[cur] / 2.0;
				mi = min(mi, L[cur]);
				done[cur] = d[cur] = true;
				cur = S[cur];
			}
			ret += mi / 2.0;
		}
	};

	function<void(int, bool)> f = [&](int i, bool top){
		if(done[i] == false){
			if(top) ret += L[i];
			else ret += L[i] / 2.0;
			done[i] = true;
		}
		EACH(c, C[i]) f(c, false);
	};

	auto checkTree = [&](int i){
		if(done[i]) return;

		int cur = i;
		while(!S[cur] == -1 && !done[cur]) cur = S[cur];
		f(i, true);
	};

	REP(i, N){
		checkLoop(i);
		checkTree(i);
	}

	cout << ret;
	if(ret == (int)ret) cout << ".0";
	cout << endl;

	return 0;
}