#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
using namespace std;

pair<int, int> dfs(int a, const vector<int>& level, vector<int>& s)
{
    if(s[a] == -1)
        return make_pair(a, level[a]);

    int b = s[a];
    s[a] = -1;
    pair<int, int> p = dfs(b, level, s);
    if(p.first == -1)
        return p;

    if(p.first == a){
        p.first = -1;
        return p;
    }

    p.second = min(p.second, level[a]);
    return p;
}

int main()
{
    int n;
    cin >> n;
    vector<int> level(n), s(n);
    for(int i=0; i<n; ++i){
        cin >> level[i] >> s[i];
        -- s[i];
    }

    int ret = accumulate(level.begin(), level.end(), 0);
    for(int i=0; i<n; ++i){
        pair<int, int> p = dfs(i, level, s);
        if(p.first == -1)
            ret += p.second;
    }

    cout << (ret / 2);
    if(ret % 2 != 0)
        cout << ".5" << endl;
    else
        cout << ".0" << endl;

    return 0;
}