#include <bits/stdc++.h>

#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define RFOR(i, a, b) for (ll i = (b)-1; i >= (a); i--)
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define rep1(i, n) for (ll i = 1; i <= (n); i++)
#define rrep(i, n) for (ll i = (n)-1; i >= 0; i--)

#define pb push_back
#define mp make_pair
#define fst first
#define snd second
#define show(x) cout << #x << " = " << x << endl
#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)
#define pii pair<int, int>
#define vi vector<int>

using namespace std;
template <class S, class T>
ostream& operator<<(ostream& o, const pair<S, T>& p)
{
    return o << "(" << p.first << "," << p.second << ")";
}
template <class T>
ostream& operator<<(ostream& o, const vector<T>& vc)
{
    o << "sz = " << vc.size() << endl
      << "[";
    for (const T& v : vc)
        o << v << ",";
    o << "]";
    return o;
}

using ll = long long;
constexpr ll MOD = 1000000007;

// Just stands for connections
class LinkList
{
public:
    LinkList(const std::size_t v) : m_v{v}
    {
        m_table.resize(v);
        m_reversed_table.resize(v);
    }
    void addEdge(const std::size_t from, const std::size_t to)
    {
        m_e++;
        m_table[from].push_back(to);
        m_reversed_table[to].push_back(from);
    }

    void TopologocalSort(std::vector<std::size_t>& srt) const
    {
        srt.clear();
        std::vector<bool> used(m_v, false);
        for (std::size_t i = 0; i < m_v; i++) {
            dfs_topo(i, used, srt);
        }
        std::reverse(srt.begin(), srt.end());
    }

    std::size_t SCC(std::vector<std::size_t>& cmp) const
    {
        assert(cmp.size() == m_v);
        for (std::size_t i = 0; i < m_v; i++) {
            cmp[i] = 0;
        }

        std::vector<std::size_t> st;
        std::vector<bool> used(m_v, false);
        for (std::size_t i = 0; i < m_v; i++) {
            if (not used[i]) {
                dfs1_scc(i, st, used);
            }
        }

        for (std::size_t i = 0; i < m_v; i++) {
            used[i] = false;
        }
        std::size_t comp = 0;
        for (std::size_t i = 0; i < st.size(); i++) {
            const std::size_t s = st[st.size() - i - 1];
            if (not used[s]) {
                dfs2_scc(s, comp++, cmp, used);
            }
        }
        return comp;
    }

    std::size_t getV() const
    {
        return m_v;
    }

    std::size_t getE() const
    {
        return m_e;
    }

    const std::vector<std::vector<std::size_t>>& getEdge() const
    {
        return m_table;
    }

    std::vector<std::vector<std::size_t>>& getEdge()
    {
        return m_table;
    }

    const std::vector<std::vector<std::size_t>>& getReversedEdge() const
    {
        return m_reversed_table;
    }

    std::vector<std::vector<std::size_t>>& getReversedEdge()
    {
        return m_reversed_table;
    }

private:
    void dfs_topo(const std::size_t s, std::vector<bool>& used, std::vector<std::size_t>& srt) const
    {
        assert(s < m_v);
        assert(used.size() == m_v);
        if (not used[s]) {
            used[s] = true;
            for (const std::size_t to : m_table[s]) {
                dfs_topo(to, used, srt);
            }
            srt.push_back(s);
        }
    }

    void dfs1_scc(const std::size_t s, std::vector<std::size_t>& st, std::vector<bool>& used) const
    {
        assert(s < m_v);
        assert(used.size() == m_v);
        used[s] = true;
        for (const std::size_t to : m_table[s]) {
            if (not used[to]) {
                dfs1_scc(to, st, used);
            }
        }
        st.push_back(s);
    }

    void dfs2_scc(const std::size_t s, const std::size_t index, std::vector<std::size_t>& cmp, std::vector<bool>& used) const
    {
        assert(s < m_v);
        assert(index < m_v);
        assert(cmp.size() == m_v);
        cmp[s] = index;
        used[s] = true;
        for (const std::size_t to : m_reversed_table[s]) {
            if (not used[to]) {
                dfs2_scc(to, index, cmp, used);
            }
        }
    };

    const std::size_t m_v;
    std::size_t m_e;
    std::vector<std::vector<std::size_t>> m_table;
    std::vector<std::vector<std::size_t>> m_reversed_table;
};

void dfs(const LinkList& g, const std::size_t pos, vector<bool>& checked)
{
    checked[pos] = true;
    for (const std::size_t to : g.getEdge()[pos]) {
        if (not checked[to]) {
            dfs(g, to, checked);
        }
    }
}

int main()
{
    std::size_t n;
    cin >> n;
    vector<int> level(n);
    int sum = 0;
    LinkList g(n);
    rep(i, n)
    {
        int l, par;
        cin >> l >> par;
        sum += l;
        level[i] = l;
        par--;
        g.addEdge(par, i);
    }
    vector<std::size_t> comp(n);
    const std::size_t size = g.SCC(comp);
    constexpr int INF = numeric_limits<int>::max() / 100;
    vector<bool> checked(n, false);
    for (int c = 0; c < size; c++) {
        int mini = INF;
        int pos = 0;
        for (int i = 0; i < n; i++) {
            if (comp[i] == c and (not checked[i])) {
                pos = i;
                mini = min(mini, level[i]);
            }
        }
        if (mini != INF) {
            dfs(g, pos, checked);
            sum += mini;
        }
    }


    cout << fixed << setprecision(1) << sum * 1.0 / 2 << endl;

    return 0;
}