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

vector<int> to;
vector<int> from;
vector<int> order;
vector<int> cmp;
vector<double> level;
bitset<110> used;

void dfs1(int x){
    used[x] = 1;
    if(!used[to[x]])dfs1(to[x]);
    order.push_back(x);
}

void dfs2(int x, int y){
    if(cmp[x]!=-1)return;
    cmp[x] = y;
    dfs2(from[x],y);
}

int main(){
    int n; cin >> n;
    to.resize(n);
    from.resize(n);
    level.resize(n);
    for (int i = 0; i < n; i++) {
        cin >> level[i] >> to[i]; to[i]--;
        from[to[i]] = i;
    }
    for(int i = 0; i < n; ++i){
        if(!used[i])dfs1(i);
    }
    cmp.resize(n,-1);
    reverse(order.begin(),order.end());
    int y = 0;
    for(int e:order){
        if(cmp[e] == -1){
            dfs2(e,y);
            y++;
        }
    }
    double sum = 0;
    for(int i = 0; i < y; ++i){
        vector<double> v;
        for(int j = 0; j < n; ++j){
            if(cmp[j] == i)v.push_back(level[j]);
        }
        sort(v.begin(),v.end());
        sum += (v.size()==1?v[0]/2:v[0]);
        for(int i = 1; i < (int)v.size(); ++i){
            sum += v[i]/2;
        }
    }
    cout << setprecision(2) << sum << endl;
    return 0;
}