#include <bits/stdc++.h>
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define FORR(i,a,b) for (int i=(a);i>=(b);i--)
#define pb push_back
#define pcnt __builtin_popcount
#define show(x) cout<<#x<<" = "<<x<<endl;
#define maxs(x,y) x = max(x,y)
#define mins(x,y) x = min(x,y)
#define fi first
#define se second
#define rng(a) (a.begin()),(a.end())
#define each(it,c) for(__typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define sz(x) (int)(x).size()
#define mp make_pair

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vpii;
typedef set<int> si;
typedef pair<ll,ll> pll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<pll> vpll;
typedef set<ll> sl;
typedef __int128_t lll;
typedef pair<lll,lll> plll;
typedef vector<lll> vlll;
template<typename T>string join(vector<T>&v)
{stringstream s;FOR(i,0,sz(v))s<<' '<<v[i];return s.str().substr(1);}
ll gcd(ll a,ll b){if(a>b)swap(a,b);for(;a>0;b%=a,swap(a,b));return b;}
int modpow(ll a,ll n,int m){if(a==0)return a;ll p=1;for(;n>0;n/=2,a=a*a%m)if(n&1)p=p*a%m;return(int)p;}
void dout(double d){printf("%.12f\n",d);}

const int iinf = 1e9;
const ll linf = 1e18;
const int mod = 1e9+7;
const double pi = acos(-1);
const double eps = 1e-10;
int n, L[100], S[100];
bool y[100];
vi m[100];
si s;
queue<int> q;

main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll ans = 0;
  cin >> n;
  FOR(i, 0, n){
    cin >> L[i] >> S[i];
    S[i]--;
    m[S[i]].pb(i);
  }
  FOR(i, 0, n){
    if(y[i]) continue;
    s.clear();
    int t = i;
    while(s.find(t) == s.end()){
      s.insert(t);
      t = S[t];
    }
    s.clear();
    int minv = iinf, idx;
    while(s.find(t) == s.end()){
      s.insert(t);
      if(minv > L[t]){
        idx = t;
        minv = L[t];
      }
      t = S[t];
    }
    q.push(idx);
    y[idx] = true;
    ans += L[idx];
    while(!q.empty()){
      t = q.front();
      q.pop();
      ans += L[t];
      each(itr, m[t]){
        if(y[*itr]) continue;
        y[*itr] = true;
        q.push(*itr);
      }
    }
  }
  printf("%.1f\n",(double)ans / 2);
  return 0;
}