#include <bits/stdc++.h>

using namespace std;

#define REP(i, n)   for(int i = 0; i < (n); i++)
#define REPS(i, n)  for(int i = 1; i <= (n); i++)
#define RREP(i, n)  for(int i = (n)-1; i >= 0; i--)
#define RREPS(i, n) for(int i = (n); i > 0; i--)
#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define mp make_pair
#define mt make_tuple
#define pb push_back

using ll  = long long;
using pi  = pair<int, int>;
using pl  = pair<ll, ll>;
using vi  = vector<int>;
using vl  = vector<ll>;
using vs  = vector<string>;
using vb  = vector<bool>;
using vvi = vector<vi>;
using vvl = vector<vl>;

const int MOD = 1e9 + 7;
const int INF = 1e9 + 7;
const ll  INFL = 1e18;
const double PI = 3.141592653589793;
const double EPS = 1e-9;

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

int N;
vector<vb> G;
vb checked;
float ans = 0;
vector<float> cost;

void dfs(int n)
{
    ans += cost[n] / 2.0;
    checked[n] = true;
    REP(i, N) if(!checked[i] && G[n][i]) dfs(i);
}

signed main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);

    cin >> N;
    G.resize(N, vb(N, false));
    cost.resize(N, 0);
    REP(i, N)
    {
        int l, s; cin >> l >> s;
        s--;
        cost[i] = l;
        G[s][i] = true;
    }

    checked.resize(N, false);

    while(true)
    {
        bool update = false;
        REP(i, N)
        {
            if(checked[i]) continue;
            bool nouse = true;
            REP(j, N)
            {
                if(G[i][j]) nouse = false;
            }
            if(nouse)
            {
                update = true;
                checked[i] = true;
                REP(j, N) G[j][i] = false;
                ans += cost[i] / 2.0;
            }
        }
        if(!update) break;
    }

    vector<pi> p;
    REP(i, N) if(!checked[i]) p.pb(mp(cost[i], i));
    sort(ALL(p));

    REP(i, (int)p.size())
    {
        int n = p[i].second;
        if(!checked[n])
        {
            ans += cost[n] / 2.0;
            dfs(n);
        }
    }
    cout << setprecision(1) << ans << endl;
}