#include <bits/stdc++.h>
using namespace std;
using ll = long long;

class dsu{
    public:
    dsu()=default;
	explicit dsu(size_t n):m_parentsOrSize(n,-1){}
	int leader(int i){
		if (m_parentsOrSize[i]<0)return i;
		return(m_parentsOrSize[i]=leader(m_parentsOrSize[i]));
	}
	int merge(int a,int b){
		a=leader(a);
		b=leader(b);
		if (a!=b){
			if(-m_parentsOrSize[a]<-m_parentsOrSize[b])std::swap(a,b);
			m_parentsOrSize[a]+=m_parentsOrSize[b];
			m_parentsOrSize[b]=a;
		}
		return leader(a);
	}
	bool same(int a,int b){return(leader(a)==leader(b));}
	int size(int i){return -m_parentsOrSize[leader(i)];}
	std::vector<std::vector<int>>groups(){
	    size_t n=m_parentsOrSize.size();
	    std::vector<std::vector<int>>A(n);
	    for(int i=0;i<n;i++)A[leader(i)].emplace_back(i);
	    std::vector<std::vector<int>>res;
	    for(auto a:A)if(a.size())res.emplace_back(a);
	    return res;
	}
    private:
	std::vector<int>m_parentsOrSize;
};
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int N;cin>>N;
    vector<ll>L(N),S(N);
    ll ans=0;
    for(int i=0;i<N;++i){
        cin>>L[i]>>S[i];
        ans+=L[i];
    }
    dsu uf(N);
    for(int i=0;i<N;++i){
        uf.merge(i,S[i]-1);
    }

    auto grps=uf.groups();
    vector<bool> vis(N);
    for(auto&g:grps){
        auto u=g[0];
        while(!vis[u]){
            vis[u]=1;
            u=S[u]-1;
        }
        vector<int> cycle;
        while(vis[u]){
            vis[u]=0;
            cycle.emplace_back(u);
            u=S[u]-1;
        }
        ll m=1e18;
        for(auto&x:cycle){
            m=min(m,L[x]);
            vis[m]=0;
        }
        ans+=m;
    }
    cout<<(ans>>1)<<((ans&1)?".5":".0")<<endl;
    return 0;
}