#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=n-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define vtpl(x,y,z) vector<tuple<x,y,z>>
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[8]={1,0,-1,0,1,1,-1,-1};
const ll dx[8]={0,-1,0,1,1,-1,1,-1};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
struct SCC{
    int n;
    vector<vector<int>> g,rg;
    vector<int> vs,cmp;
    vector<bool> used;
    SCC(int _n){
        n=_n;
        g=vector<vector<int>>(n);rg=g;
        cmp=vector<int>(n);used=vector<bool>(n);
    }
    void add_edge(int f, int t){
        g[f].push_back(t);
        rg[t].push_back(f);
    }
    int init(){
        rep(i,n)if(!used[i])dfs(i);
        int k=0;
        used=vector<bool>(n,false);
        per(i,n){
            if(!used[vs[i]]){
                rdfs(vs[i],k);
                k++;
            }
        }
        return k;
    }
    private:
    void dfs(int v){
        used[v]=true;
        rep(i,g[v].size()){
            if(!used[g[v][i]])dfs(g[v][i]);
        }
        vs.push_back(v);
    }
    void rdfs(int v, int k){
        used[v]=true;cmp[v]=k;
        rep(i,rg[v].size()){
            if(!used[rg[v][i]]){
                rdfs(rg[v][i],k);
            }
        }
    }
};

int main(){
    ll n;cin >> n;
    SCC g(n);
    vector<double> cost(n);
    vector<pl> edge(n);
    rep(i,n){
        ll a,b;cin >> a >> b;b--;
        cost[i]=a;
        g.add_edge(b,i);
        edge[i]={b,i};
    }
    int m=g.init();
    vvl pos(m);
    vvl ng(m);
    for(auto p:edge){
        if(g.cmp[p.fi]==g.cmp[p.se])continue;
        ng[g.cmp[p.se]].pb(g.cmp[p.fi]);
    }
    rep(i,n){
        pos[g.cmp[i]].pb(i);
    }
    double ans=0;
    rep(i,m){
        double z=inf;
       for(auto p:pos[i]){
           ans+=cost[p]/2;
           chmin(z,cost[p]);
       }
       if(ng[i].size()==0){
           ans+=z/2;
       }
    }
    CST(1);
    cout << ans <<endl;
}