#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;

vi G[100], stk;
double cost[100][100], ans = 0;
int N, vis[100];

void dfs(int u){
    if(vis[u] == 1){
        stk.push_back(u);
        double mini = 1e10;
        for(int i = sz(stk) - 2; i >= 0; --i){
            smin(mini, cost[stk[i]][stk[i + 1]]);
        }
        ans += mini;
        stk.pop_back();
    }
    if(vis[u])return;
    vis[u] = 1;
    stk.push_back(u);
    each(v, G[u]){
        dfs(v);
    }
    vis[u] = 2;
    stk.pop_back();
}

int main(){
    cin >> N;
    rep(i, N){
        int L, S;
        cin >> L >> S;
        ans += L *0.5;
        G[--S].push_back(i);
        cost[S][i] = L*0.5;
    }
    rep(i, N)dfs(i);
    printf("%0.1f\n", ans);
}