#include <algorithm>
#include <cmath>
#include <cstring>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <iterator>

//#define pb push_back
#define puts(x) cout << #x << " : " << x << endl;
//#pragma GCC diagnostic ignored "-Wconversion"
//#define REP(i,n) for (int i=0;i<(n);i++)
//#define REPE(i,n) for (int i=0;i<=(n);i++)
//#define init(a,b) memset((a), (b), (sizeof(a)));
//#define PI 3.14159265
//#define EPS (1e-10)
//#define EQ(a,b) (abs((a)-(b)) < EPS)

using namespace std;

const int INF = 1000*1000;

typedef long long ll;
//#define int long long

int dxy[] = {0, 1, 0, -1, 0};

typedef pair<int, int> P;

int l[105],s[105];

const int MAX_V = 105;
int V;
vector<int> G[MAX_V];
vector<int> rG[MAX_V];
vector<int> vs;
bool used[MAX_V];
int cmp[MAX_V];

void add_edge(int from, int to) {
    G[from].push_back(to);
    rG[to].push_back(from);
}

void dfs(int v){
    used[v] = true;
    for (int i = 0; i < G[v].size(); i++) {
        if (!used[G[v][i]]) dfs(G[v][i]);
    }
    vs.push_back(v);
}
void rdfs(int v, int k) {
    used[v] = true;
    cmp[v] = k;
    for (int i = 0; i < rG[v].size(); i++) {
        if (!used[rG[v][i]])
            rdfs(rG[v][i], k);
    }
}
int scc() {
    memset(used, 0, sizeof(used));
    vs.clear();
    for (int v = 0; v < V; v++) {
        if (!used[v]) dfs(v);
    }
    memset(used, 0, sizeof(used));
    int k = 0;
    for (int i = vs.size() - 1; i >= 0; i--) {
        if (!used[vs[i]]){
            rdfs(vs[i], k++);
        }
    }
    return k;
}

signed main() {
    cin>>V;
    double res = 0;
    for(int i=0;i<V;i++){
        cin>>l[i]>>s[i];
        s[i]--;
        add_edge(s[i], i);
    }
    int cnt = scc();
    for(int k = 0; k < cnt; k++){
        bool f = false;
        int mn = INF;
        for(int i=0;i<V;i++){
            if (cmp[i] != k) continue;
            // このkに対して<kからのパスがある = 違うkに属してるs[i]がある
            if (cmp[s[i]] < k) f = true;
            mn = min(mn, l[i]);
        }
        for(int i=0;i<V;i++){
            if (cmp[i] != k) continue;
            res += l[i]/2.0;
        }
        if(!f){
            res += mn / 2.0;
        }
    }
    printf("%.1lf\n", res);
    return 0;
}