#include <iostream>
#include <vector>
#include <queue>

void solve() {
    int n;
    std::cin >> n;

    int sum = 0;
    std::vector<int> xs(n), vs(n), indeg(n, 0);
    for (int i = 0; i < n; ++i) {
        std::cin >> xs[i] >> vs[i];
        sum += xs[i];
        ++indeg[--vs[i]];
    }

    std::queue<int> que;
    for (int v = 0; v < n; ++v) {
        if (indeg[v] == 0) que.push(v);
    }

    while (!que.empty()) {
        int v = que.front();
        que.pop();
        xs[v] = -1;

        int u = vs[v];
        if (--indeg[u] == 0) que.push(u);
    }

    for (int s = 0; s < n; ++s) {
        if (xs[s] == -1) continue;
        int min = xs[s];

        int v = s;
        while (vs[v] != s) {
            v = vs[v];
            min = std::min(min, xs[v]);
            xs[v] = -1;
        }

        sum += min;
    }

    std::cout << sum / 2 << "." << sum % 2 * 5 << "\n";
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}