#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001



int main(){
	
	int N;
	cin>>N;
	
	vector<int> l(N),s(N);
	
	scc_graph S(N);
	rep(i,N){
		cin>>l[i]>>s[i];
		s[i]--;
		S.add_edge(s[i],i);
		l[i] *= 2;
	}
	
	int ans = 0;
	auto g = S.scc();
	
	rep(i,g.size()){
		vector<int> t;
		rep(j,g[i].size()){
			t.push_back(l[g[i][j]]);
		}
		if(t.size()==1){
			if(s[g[i][0]]==g[i][0])ans += l[g[i][0]];
			else ans += l[g[i][0]]/2;
		}
		else{
			sort(t.begin(),t.end());
			rep(j,t.size()){
				if(j==0)ans += t[j];
				else ans += t[j]/2;
			}
		}
	}
	
	cout<<ans/2;
	if(ans%2==1)cout<<".5";
	else cout<<".0";
	cout<<endl;
	
	return 0;
}