#include<bits/stdc++.h> using namespace std; using ll = long long; int main(){ cin.tie(nullptr); ios::sync_with_stdio(false); int n; cin>>n; vector<double> l(n); vector<int> u(n); double ans = 0; vector<int> cnt(n,0); vector<int> vis(n,0); for(int i = 0;i<n;i++){ cin>>l[i]>>u[i]; u[i]--; cnt[u[i]]++; if(u[i]==i){ ans += l[i]; vis[i]++; } } vector<int> que; for(int i = 0;i<n;i++) if(cnt[i]==0) que.push_back(i); for(int i = 0;i<que.size();i++){ int ni = que[i]; vis[ni]++; ans += l[ni] / 2; cnt[u[ni]]--; if(cnt[u[ni]]==0) que.push_back(u[ni]); } for(int i = 0;i<n;i++){ if(vis[i]) continue; double mn = 1e9; int ni = i; while(true){ mn = min(mn,l[ni]); vis[ni]++; ans += l[ni] / 2.; ni = u[ni]; if(ni==i) break; } ans += mn/2.; } cout<<setprecision(1)<<fixed<<ans<<endl; }