#include <bits/stdc++.h> 

#define int long long
#define double long double

#define REP(i, b) for(int i = 0; i < (b); i++)
#define REPS(i, b) for(int i = 1; i <= (b); i++)
#define ALL(v) (v).begin(), (v).end()

#define fi first
#define se second
#define pb push_back
#define mp make_pair

using namespace std;

using pi = pair<int, int>;
using vi = vector<int>;
using vd = vector<double>;
using vs = vector<string>;
using vb = vector<bool>;
using vpi = vector<pi>;
using vvi = vector<vi>;
using vvb = vector<vb>;

const int INF = 1e16;
const int MOD = 1e9+7;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

vi node;

int root(int x)
{
    if(node[x] == x) return x;
    return node[x] = root(node[x]);
}

void unite(int x, int y)
{
    int rx = root(x);
    int ry = root(y);
    node[rx] = ry;
}

signed main()
{
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(1);

    int N; cin >> N;
    vector<pair<double, pair<int, int>>> s(N+1);
    s[0].fi = INF;
    vd L(N+1);
    REPS(i, N)
    {
        cin >> L[i];
        int to; cin >> to;
        s[i].fi = L[i];
        s[i].se = mp(to, i);
    }
    sort(ALL(s), greater<>());
    #ifdef DEBUG
        cout << endl;
        REPS(i, N) cout << s[i].se.fi << " " << s[i].se.se << endl;
        cout << endl;
    #endif
    node.resize(N+1); REPS(i, N) node[i] = i;
    REPS(i, N)
    {
        if(root(s[i].se.fi) == s[i].se.se) continue;
        unite(s[i].se.se, s[i].se.fi);
    }
    #ifdef DEBUG
        REPS(i, N) cout << root(i) << " ";
        cout << endl;
    #endif
    double ans = 0;
    REPS(i, N)
    {
        if(root(i) == i) ans += L[i];
        else ans += L[i]/2;
    }
    cout << ans << endl;
}