#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
#include <functional>
#define repeat(i,n) for (int i = 0; (i) < int(n); ++(i))
#define whole(f,x,...) ([&](decltype((x)) whole) { return (f)(begin(whole), end(whole), ## __VA_ARGS__); })(x)
using namespace std;
int main() {
    // input
    int n; cin >> n;
    vector<int> l(n);
    vector<vector<int> > g(n);
    vector<int> g_inv(n);
    repeat (i,n) {
        int s; cin >> l[i] >> s; -- s;
        g[s].push_back(i);
        g_inv[i] = s;
    }
    // solve
    double ans = whole(accumulate, l, 0) / 2.0;
    vector<bool> used(n);
    function<void (int)> use = [&](int i) {
        if (used[i]) return;
        used[i] = true;
        for (int j : g[i]) if (not used[j]) use(j);
    };
    function<void (int, vector<int> &)> roots = [&](int i, vector<int> & acc) {
        auto it = whole(find, acc, i);
        if (it != acc.end()) {
            acc.erase(acc.begin(), it);
        } else {
            acc.push_back(i);
            roots(g_inv[i], acc);
        }
    };
    repeat (i,n) if (not used[i]) {
        vector<int> acc;
        roots(i, acc);
        int j = *whole(min_element, acc, [&](int i, int j) { return l[i] < l[j]; });
        use(j);
        ans += l[j] / 2.0;
    }
    // output
    printf("%.1f\n", ans);
    return 0;
}