#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) __typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int,int> pii; typedef vector<pair<int,int> > vpii; typedef long long ll;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

void findCycles(const vector<int> &f, vector<int> &cyclerep) {
	int n = f.size();
	vector<bool> vis(n);
	vi path; path.reserve(n);
	cyclerep.assign(n, -1);
	rep(i, n) if(!vis[i]) {
		path.clear();
		int j = i;
		while(j != -1 && !vis[j]) {
			path.push_back(j);
			vis[j] = true;
			j = f[j];
		}
		int k = find(all(path), j) - path.begin();
		if(k != path.size()) {
			for(; k != path.size(); ++ k)
				cyclerep[path[k]] = j;
		}
	}
}

int main() {
	int N;
	while(cin >> N) {
		vector<int> L(N), S(N);
		rep(i, N) cin >> L[i] >> S[i], -- S[i];
		vi cyclerep;
		findCycles(S, cyclerep);
		vector<int> mini(N, INF), sum(N, 0);
		int ans = 0;
		rep(i, N) {
			int r = cyclerep[i];
			if(r != -1) {
				amin(mini[r], L[i]);
				sum[r] += L[i];
			}else {
				ans += L[i];
			}
		}
		rep(i, N) if(cyclerep[i] == i)
			ans += sum[i] + mini[i];

		printf("%d.%d\n", ans / 2, ans % 2 * 5);
	}
	return 0;
}