#include<bits/stdc++.h>

#define debug(x) cerr << #x << ": " << x << '\n'
#define debugArray(x,n) for(long long hoge = 0; (hoge) < (n); ++ (hoge)) cerr << #x << "[" << hoge << "]: " << x[hoge] << '\n'
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<ll> vll;
const ll INF = INT_MAX;
const ll MOD = 20171211;

ll N;
ll L[110],S[110];
ll visit[110];

ll dfs(ll v){
  if(visit[v]==2)return 0;
  if(visit[v]==1){
    visit[v]=2;
    return L[v];
  }
  visit[v]=1;
  ll ret=dfs(S[v]);
  if(visit[v]==2){
    return -ret;
  }
  visit[v]=2;
  if(ret<=0)return ret;
  ret = min(ret,L[v]);
  return ret;

}

signed main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll N;cin>>N;
  for(ll i=0;i<N;i++){
    cin>>L[i]>>S[i];S[i]--;
  }
  double ans = 0;
  for(ll i=0;i<N;i++){
    ans -= dfs(i);
    ans += L[i];
  }
  ans /= 2;
  printf("%.1lf\n",ans);
  return 0;
}