#include <algorithm>
#include <iostream>
#include <vector>

int main() {
    using namespace std;
    int n;
    cin >> n;
    vector<int> level(n), stk;
    vector<vector<int>> g(n);
    for (int i = 0; i < n; i++) {
        int l, s;
        cin >> l >> s;
        level[i] = l;
        if (i == s - 1) {
            stk.push_back(i);
        } else {
            g[s - 1].push_back(i);
        }
    }

    vector<bool> cleared(n, false), half(n, false);
    int ans = 0;
    while (!stk.empty()) {
        int v = stk.back();
        stk.pop_back();
        ans += (half[v] ? level[v] : level[v] * 2);
        cleared[v] = true;
        for (int nv : g[v]) {
            if (cleared[nv]) { continue; }
            stk.push_back(nv);
            half[nv] = true;
        }
    }

    for (int i = 0; i < n; i++) {
        if (cleared[i]) { continue; }
        vector<int> lv, st = {i};
        while (!st.empty()) {
            int v = st.back();
            st.pop_back();
            cleared[v] = true;
            lv.push_back(level[v]);
            for (int nv : g[v]) {
                if (cleared[nv]) { continue; }
                st.push_back(nv);
            }
        }
        sort(lv.begin(), lv.end());
        ans += lv[0];
        for (int e : lv) { ans += e; }
    }

    cout << ans / 2 << '.' << (ans % 2 == 0 ? 0 : 5) << endl;
}