#include <algorithm>
#include <iostream>
#include <vector>
#include <set>
using namespace std;
const int INF = (int)1e9 + 7;
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

struct StronglyConnectedComponents {
    vector<vector<int>> gg, rg;
    vector<int> comp, order, seen;
    StronglyConnectedComponents(int v) : gg(v), rg(v), comp(v, -1), seen(v, 0) { }
    void add_edge(int x, int y) {
        gg[x].emplace_back(y);
        rg[y].emplace_back(x);
    }
    void dfs(int u) {
        seen[u] = true;
        for (int v: gg[u]) if (!seen[v]) dfs(v);
        order.emplace_back(u);
    }
    void rdfs(int u, int id) {
        comp[u] = id;
        for (int v: rg[u]) if (comp[v] == -1) rdfs(v, id);
    }
    vector<vector<int>> build() {
        for (int u = 0; u < gg.size(); ++u) if (!seen[u]) dfs(u);
        reverse(begin(order), end(order));
        int id = 0;
        for (int u: order) if (comp[u] == -1) rdfs(u, id++);
        vector<vector<int>> g_comp(id);
        set<pair<int, int>> connect;
        for (int u = 0; u < gg.size(); ++u) for (int v: gg[u]) {
            int x = comp[u], y = comp[v];
            if (x == y) continue;
            if (connect.count({x, y})) continue;
            g_comp[x].emplace_back(y);
            connect.emplace(x, y);
        }
        return g_comp;
    }
    int operator[](int k) { return comp[k]; }
};

#include <iomanip>
int main() {
    //cout << fixed << setprecision(1);
    int n; cin >> n;
    vector<int> lv(n);
    StronglyConnectedComponents scc(n);
    double ans = 0;
    for (int i = 0; i < n; i++) {
        int s; cin >> lv[i] >> s; s--;
        ans += lv[i];
        scc.add_edge(s, i);
    }
    ans /= 2;
    vector<vector<int>> dag = scc.build();
    int sz = dag.size();
    vector<int> in_deg(sz);
    for (int u = 0; u < sz; u++) for (auto v: dag[u]) in_deg[v]++;
    vector<int> mins(sz, INF);
    for (int i = 0; i < n; i++) if (in_deg[scc[i]] == 0) {
        chmin(mins[scc[i]], lv[i]);
    }
    for (int u = 0; u < sz; u++) if (mins[u] != INF) ans += 1.0 * mins[u] / 2;
    cout << ans << endl;
    return 0;
}