#include <atcoder/scc>
#include <algorithm>
#include <iostream>
#include <utility>
#include <vector>

int main() {
    using namespace std;
    int n;
    cin >> n;
    vector<pair<int, int>> edges;
    atcoder::scc_graph scc(n);
    vector<int> level(n);
    for (int i = 0; i < n; i++) {
        int l, s;
        cin >> l >> s;
        edges.emplace_back(s - 1, i);
        scc.add_edge(s - 1, i);
        level[i] = l;
    }

    vector<vector<int>> groups = scc.scc();
    vector<int> group_id(n);
    int m = groups.size();
    for (int i = 0; i < m; i++) {
        for (int v : groups[i]) { group_id[v] = i; }
    }
    vector<int> indeg(m, 0);
    for (auto [u, v] : edges) {
        if (group_id[u] == group_id[v]) { continue; }
        indeg[group_id[v]]++;
    }

    int ans = 0;
    for (int i = 0; i < m; i++) {
        int lv_sum = 0, min_lv = 1000;
        for (int v : groups[i]) {
            lv_sum += level[v];
            min_lv = min(min_lv, level[v]);
        }
        ans += lv_sum;
        if (indeg[i] == 0) { ans += min_lv; }
    }
    cout << ans / 2 << '.' << (ans % 2 == 0 ? 0 : 5) << endl;
}