#include <iostream>
#include <vector>
using namespace std;

int main(void) {
    int N;
    cin >> N;
    int L[N], S[N];
    for (int i = 0; i < N; i++) {
        int level, stage;
        cin >> level >> stage;
        L[i] = level << 1;
        S[i] = stage - 1;
    }
    vector<int> children[N];
    int tansaku[N] = {0};
    for (int i = 0; i < N; i++) {
        children[S[i]].push_back(i);
    }
    int graph_id = 1;
    int sum = 0;
    for (int i = 0; i < N; i++) {
        if (tansaku[i] == 0) {
            int takinobori[N] = {0};
            vector<int> passed;
            takinobori[i] = 1;
            passed.push_back(i);
            int cur = i, next, kon;
            for (;1;) {
                next = S[cur];
                if (next == cur) {
                    kon = cur;
                    break;
                } else if (takinobori[next] == 1) {
                    int min = L[next];
                    int minver = next;
                    int found = 0;
                    int lsize = passed.size();
                    for (int j = 0; j < lsize; j++) {
                        if (found) {
                            if (L[passed[j]] < min) {
                                minver = passed[j];
                                min = L[minver];
                            }
                        }else if (passed[j] == next) {
                            found = 1;
                        }
                    }
                    kon = minver;
                    break;
                } else {
                    takinobori[next] = 1;
                    passed.push_back(next);
                    cur = next;
                }
            }
            vector<int> que = {kon};
            tansaku[kon] = graph_id;
            sum += L[kon];
            for (;que.size();) {
                int cur = que[0];
                que.erase(que.begin());
                int csize = children[cur].size();
                for (int j = 0; j < csize; j++) {
                    int child = children[cur][j];
                    if (tansaku[child] == 0) {
                        tansaku[child] = graph_id;
                        que.push_back(child);
                        sum += L[child] / 2;
                    }
                }
            }
            graph_id++;
        }
    }

    cout << sum / 2 << "." << (sum % 2)*5 << endl;
    return 0;
}