/*** author: yuji9511 ***/
#include <bits/stdc++.h>
// #include <atcoder/all>
// using namespace atcoder;
using namespace std;
using ll = long long;
using lpair = pair<ll, ll>;
using vll = vector<ll>;
const ll MOD = 1e9+7;
const ll INF = 1e18;
#define rep(i,m,n) for(ll i=(m);i<(n);i++)
#define rrep(i,m,n) for(ll i=(m);i>=(n);i--)
#define printa(x,n) for(ll i=0;i<n;i++){cout<<(x[i])<<" \n"[i==n-1];};
void print() {}
template <class H,class... T>
void print(H&& h, T&&... t){cout<<h<<" \n"[sizeof...(t)==0];print(forward<T>(t)...);}

using UnWeightedGraph = vector<vll>;
template< typename G >
struct SCC {
	const G &g;
	UnWeightedGraph gg, rg;
	vll comp, order, used;

	SCC(G &g) : g(g), gg(g.size()), rg(g.size()), comp(g.size(), -1), used(g.size()) {
		for(int i = 0; i < g.size(); i++) {
			for(auto e : g[i]) {
				gg[i].emplace_back((int) e);
				rg[(int) e].emplace_back(i);
			}
		}
	}

	int operator[](int k) {
		return comp[k];
	}

	void dfs(int idx) {
		if(used[idx]) return;
		used[idx] = true;
		for(int to : gg[idx]) dfs(to);
		order.push_back(idx);
	}

	void rdfs(int idx, int cnt) {
		if(comp[idx] != -1) return;
		comp[idx] = cnt;
		for(int to : rg[idx]) rdfs(to, cnt);
	}

	void build(UnWeightedGraph &t) {
		for(int i = 0; i < gg.size(); i++) dfs(i);
		reverse(begin(order), end(order));
		int ptr = 0;
		for(int i : order) if(comp[i] == -1) rdfs(i, ptr), ptr++;

		t.resize(ptr);
		for(int i = 0; i < g.size(); i++) {
			for(auto &to : g[i]) {
				int x = comp[i], y = comp[to];
				if(x == y) continue;
				t[x].push_back(y);
			}
		}
	}
};

struct UnionFind {
private:
    ll N;
    vector<ll> parent;
    vector<ll> num;
    vector<ll> diff_weight;

public:
    UnionFind(ll n){
        N = n;
        parent.resize(N);
        iota(parent.begin(), parent.end(), 0);
        num.assign(N, 1);
        diff_weight.assign(N, 0);
    }

    ll root(ll x){
        if(x == parent[x]){
            return x;
        }else{
            ll r = root(parent[x]);
            diff_weight[x] += diff_weight[parent[x]];
            return parent[x] = r;
        }
    }

    void unite(ll a, ll b, ll w = 0){
        w += weight(a); w -= weight(b);
        a = root(a); b = root(b);
        if(a == b) return;
        parent[b] = a;
        ll sum = num[a] + num[b];
        num[a] = sum;
        num[b] = sum;
        diff_weight[b] = w;
    }

    bool same(ll a, ll b){ return root(a) == root(b);}
    
    ll sz(ll x){ return num[root(x)];}

    ll weight(ll x){
        root(x);
        return diff_weight[x];
    }

    ll diff(ll a, ll b){
        return weight(b) - weight(a);
    }
};



void solve(){
    ll N;
    cin >> N;
    vll L(N), S(N);
    UnWeightedGraph g(N);
    UnionFind uf(N);
    map<ll,bool> loop;

    rep(i,0,N){
        cin >> L[i] >> S[i];
        S[i]--;
        g[S[i]].push_back(i);
        uf.unite(i, S[i]);
        if(i == S[i]) loop[i] = true;
    }

    UnWeightedGraph tree;
    SCC scc(g);
    scc.build(tree);
    double ans = 0.0;
    vll min_val(N+1, INF);
    vll cnt(N+1, 0);
    rep(i,0,N) cnt[scc[i]]++;
    rep(i,0,N){
        if(cnt[scc[i]] == 1 && loop[i] == false) continue;
        min_val[scc[i]] = min(min_val[scc[i]], L[i]);
    }
    rep(i,0,N){
        ans += (double) L[i] / 2.0;
    }

    rep(i,0,N){
        if(min_val[i] != INF){
            ans +=(double) min_val[i] / 2.0;
        }
    }
    printf("%.1f\n", ans);


}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	solve();
}