////////////////////////////////////////
///  tu3 pro-con template            ///
////////////////////////////////////////
#include <cassert>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <numeric>
#include <functional>
#include <vector>
#include <queue>
#include <string>
#include <complex>
#include <stack>
#include <set>
#include <map>
#include <list>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <regex>
#include <type_traits>
#include <mutex>
using namespace std;

//// MACRO ////
#define countof(a) (sizeof(a)/sizeof(a[0]))

#define REP(i,n) for (int i = 0; i < (n); i++)
#define RREP(i,n) for (int i = (n)-1; i >= 0; i--)
#define FOR(i,s,n) for (int i = (s); i < (n); i++)
#define RFOR(i,s,n) for (int i = (n)-1; i >= (s); i--)
#define pos(c,i) c.being() + (i)
#define allof(c) c.begin(), c.end()
#define aallof(a) a, countof(a)
#define partof(c,i,n) c.begin() + (i), c.begin() + (i) + (n)
#define apartof(a,i,n) a + (i), a + (i) + (n)
#define long long long

#define EPS 1e-9
#define INF (1L << 20)
#define LINF (1LL << 60)

#define PREDICATE(t,a,exp) [&](const t & a) -> bool { return exp; }
#define COMPARISON_T(t) bool(*)(const t &, const t &)
#define COMPARISON(t,a,b,exp) [&](const t & a, const t & b) -> bool { return exp; }
#define CONVERTER(TSrc,t,TDest,exp) [&](const TSrc &t)->TDest { return exp; }

inline int sign_of(double x) { return (abs(x) < EPS ? 0 : x > 0 ? 1 : -1); }
inline bool inRange(int val, int min, int max) { return val >= min && val < max; }
inline bool inRange(double val, double min, double max) { return val - min > -EPS && val - max < EPS; }
inline bool inRange(int x, int y, int W, int H) { return x >= 0 && x < W && y >= 0 && y < H; } // W,H含まない

template <class T> struct vevector : public vector<vector<T>> { vevector(int n = 0, int m = 0, const T &initial = T()) : vector<vector<T>>(n, vector<T>(m, initial)) { } };
template <class T> struct vevevector : public vector<vevector<T>> { vevevector(int n = 0, int m = 0, int l = 0, const T &initial = T()) : vector<vevector<T>>(n, vevector<T>(m, l, initial)) { } };
template <class T> struct vevevevector : public vector<vevevector<T>> { vevevevector(int n = 0, int m = 0, int l = 0, int k = 0, const T &initial = T()) : vector<vevevector<T>>(n, vevevector<T>(m, l, k, initial)) { } };

//// i/o helper ////

namespace std {
template <class T1, class T2> inline istream & operator >> (istream & in, pair<T1, T2> &p) { in >> p.first >> p.second; return in; }
template <class T1, class T2> inline ostream & operator << (ostream &out, const pair<T1, T2> &p) { out << p.first << " " << p.second; return out; }
}
template <class T> T read() { T t; cin >> t; return t; }
template <class T> vector<T> read(int n) { vector<T> v; REP(i, n) { v.push_back(read<T>()); } return v; }
template <class T> vevector<T> read(int n, int m) { vevector<T> v; REP(i, n) v.push_back(read<T>(m)); return v; }
template <class T> vector<T> readjag() { return read<T>(read<int>()); }
template <class T> vevector<T> readjag(int n) { vevector<T> v; REP(i, n) v.push_back(readjag<T>()); return v; }

template <class T> struct iter_pair_t { T beg, end; };
template <class T> iter_pair_t<T> iter_pair(T beg, T end) { return iter_pair_t<T>{beg, end}; }
template <class T> ostream & operator << (ostream &out, const iter_pair_t<T> &v)
{
	std::copy(v.beg, v.end, ostream_iterator<typename decltype(v.beg)::reference>(out, " "));
	return out;
}
template <class T1> ostream & operator << (ostream &out, const vector<T1> &v) { return out << iter_pair(begin(v), end(v)); }
template <class T1> ostream & operator << (ostream &out, const set<T1> &v) { return out << iter_pair(begin(v), end(v)); }
template <class T1, class T2> ostream & operator << (ostream &out, const map<T1,T2> &v) { return out << iter_pair(begin(v), end(v)); }

struct _Reader { istream &cin;template <class T> _Reader operator ,(T &rhs) { cin >> rhs; return *this; } };
struct _Writer { ostream &cout; bool f{false}; template <class T> _Writer operator ,(const T &rhs) { cout << (f ? " " : "") << rhs; f = true; return *this; } };
#define READ(t,...) t __VA_ARGS__; (_Reader{cin}), __VA_ARGS__
#define WRITE(...) (_Writer{cout}), __VA_ARGS__; cout << endl
#define DEBUG(...) (_Writer{cerr}), __VA_ARGS__; cerr << endl

void solve();
int main()
{
	cin.tie(0);
	ios_base::sync_with_stdio(false);
	solve();
	return 0;
}

// warshall_floyd //
template <class COST>
void warshall_floyd(vector<vector<COST>> &cost)
{
	size_t n = cost.size();
	REP(k, n) REP(i, n) REP(j, n) cost[i][j] = min(cost[i][j], cost[i][k] + cost[k][j]);
}

/// ユニオンファインド森。ユニオンファインドが必要な問題が解ける。
struct UnionFindForest
{
	vector<int> p;
	UnionFindForest(int n) : p(n, -1) { }
	int rootOf(int i) { return p[i] < 0 ? i : (p[i] = rootOf(p[i])); }
	int countOf(int i) { return -p[rootOf(i)]; }

	bool linked(int a, int b) { return rootOf(a) == rootOf(b); }
	bool link(int a, int b)
	{
		int x = rootOf(a), y = rootOf(b);
		if (x != y)
		{
			p[x] += p[y];
			p[y] = x;
		}
		return x != y;
	}
};
////////////////////
/// template end ///
////////////////////

void solve()
{
	READ(int, N);
	auto S = read<pair<int, int>>(N);

	auto calcScore = [&](const vector<int> ans) -> int
	{
		int score = 0;
		auto half = vector<bool>(N + 1, false);
		for (auto k : ans)
		{
			if (half[S[k].second - 1]) { score += S[k].first; }
			else { score += S[k].first * 2; }
			half[k] = true;
		}
		return score;
	};

	vevector<int> route = vevector<int>(N, N, 1);
	REP(i,N)
	{
		route[S[i].second - 1][i] = 0;
	}
	warshall_floyd(route);
	vevector<int> ren;

	REP(i, N)
	{
		bool found = false;
		REP(j, ren.size())
		{
			int r = ren[j][0];
			if (route[i][r] == 0 && route[r][i] == 0)
			{
				ren[j].push_back(i);
				found = true;
				break;
			}
		}
		if(!found)
		{
			ren.push_back({ i });
		}
	}

	REP(i, ren.size())
	{
		auto x = COMPARISON(vector<int>, a, b, route[a[0]][b[0]] == 0);
		REP(j, ren.size())
		{
			if (x(ren[i], ren[j]))
			{
				ren[i].swap(ren[j]);
			}
		}
	}

	int score = 0;
	REP(i, ren.size())
	{
		bool allHangaku = false;
		FOR(j, 0, i)
		{
			allHangaku |= route[ren[j][0]][ren[i][0]] == 0;
		}


		auto it = min_element(allof(ren[i]), COMPARISON(int, a, b, S[a].first < S[b].first));
		swap(*ren[i].begin(), *it);

		if (allHangaku)
		{
			score += S[ren[i][0]].first;
		}
		else
		{
			score += S[ren[i][0]].first * 2;
		}

		FOR(j, 1, ren[i].size())
		{
			score += S[ren[i][j]].first;
		}
	}

	printf("%3.1f\n", score / 2.0);

}