#include <cstdio>
#include <iostream>
#include <vector>
#include <queue>

std::vector<int> G[100], rG[100], vs;
bool used[100], used2[100];
int L[100];

void dfs(int u){
    used[u] = true;

    for(int v : G[u]){
        if(!used[v]){dfs(v);}
    }
    
    vs.push_back(u);
}

int main(){
    int N;
    scanf("%d", &N);

    for(int i=0;i<N;i++){
        int s;
        scanf("%d %d", L+i, &s);
        s -= 1;

        if(i == s){continue;}
        G[s].push_back(i);
        rG[i].push_back(s);
    }

    for(int i=0;i<N;i++){
        if(!used[i]){dfs(i);}
    }

    // for(int v : vs){printf("%d ", v);}
    // puts("");

    std::fill(used, used+N, false);

    int res = 0;
    for(int i=N-1;i>=0;i--){
        if(used[vs[i]]){continue;}

        int mn = 1001001001;

        std::fill(used2, used2+N, false);

        std::queue<int> q;
        q.push(vs[i]);
        used2[vs[i]] = true;
        mn = L[vs[i]];
        
        while(!q.empty()){
            int u = q.front(); q.pop();
            
            for(int v : rG[u]){
                if(!used2[v]){
                    used2[v] = true;
                    mn = std::min(mn, L[v]);
                    q.push(v);
                }
            }
        }

        int sum = 0;

        q.push(vs[i]);
        used[vs[i]] = true;
        sum += L[vs[i]];

        while(!q.empty()){
            int u = q.front(); q.pop();
            
            for(int v : G[u]){
                if(!used[v]){
                    used[v] = true;
                    sum += L[v];
                    q.push(v);
                }
            }
        }

        // printf("%d, %d\n", sum, mn);
        res += sum + mn;
    }

    printf("%.1f\n", (double)res / 2);
}