#include <iostream>
#include <sstream>
#include <math.h>
#include <vector>
#include <array>
#include <algorithm>
#include <queue>
#include <numeric>
#include <map>
#include <set>
#include <bitset>

using namespace std;
#define int long long
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define RFOR(i, a, b) for(int i=(b-1);i>=(a);i--)
#define REP(i, n) for(int i=0; i<(n); i++)
#define RREP(i, n) for(int i=(n-1); i>=0; i--)
#define ALL(a) (a).begin(),(a).end()
#define UNIQUE_SORT(l) sort(ALL(l)); l.erase(unique(ALL(l)), l.end());
#define CONTAIN(a, b) find(ALL(a), (b)) != (a).end()
#define SUM(a) accumulate(ALL(a), 0)
#define array2(type, x, y) array<array<type, y>, x>
#define vector2(type) vector<vector<type> >
#define out(...) printf(__VA_ARGS__)

typedef pair<int, int> pos;
int pos::*x = &pos::first;
int pos::*y = &pos::second;

int dxy[] = {0, 1, 0, -1, 0};

/*================================*/

int N;

struct stage {
    int i, lv, before;
    bool cleard;
    stage *b;
};

vector<stage> stages;

signed main()
{
    cin >> N;
    int L, S;
    REP(i,N) {
        cin >> L >> S;
        stage s = stage();
        s.i = i;
        s.lv = L;
        s.before = S - 1;
        s.cleard = false;
        stages.push_back(s);
    }
    REP(i,N) {
        stage s = stages[i];
        stages[i].b = &(stages[s.before]);
    }
    double sumlv = 0;
    REP(i,N) {
        vector<int> vis;
        stage s = stages[i];
        if (s.cleard) continue;
        while(true) {
            if (CONTAIN(vis, s.i)) {
                break;
            }
            vis.push_back(s.i);
            s = *s.b;
        }
        vector<int> loop;
        bool start = false;
        for (int j : vis) {
            if (!start && j!=s.i) continue;
            else start = true;
            loop.push_back(j);
        }
        int minLvi = 1e3;
        int minLv = 1e3;
        for (int j : loop) {
            if (minLv > stages[j].lv) {
                minLv = stages[j].lv;
                minLvi = j;
            }
        }
        for (int j : vis) {
            if (stages[j].cleard) continue;
            if (j == minLvi) {
                sumlv += stages[j].lv;
            } else {
                sumlv += stages[j].lv / 2.0;
            }
            stages[j].cleard = true;
        }
    }
    
    printf("%.1f\n", sumlv);
    
    return 0;
}