import sys
input = sys.stdin.readline
from collections import *

def bfs(s, g):
    q = deque([s])
    dist = [-1]*N
    dist[s] = 0
    
    while q:
        v = q.popleft()
        
        for nv in G[v]:
            if dist[nv]==-1:
                dist[nv] = dist[v]+1
                q.append(nv)
    
    return dist[g]!=-1
    
N = int(input())
ans = 0.0
decr = []

for i in range(N):
    L, S = map(int, input().split())
    ans += L
    decr.append((L/2, S-1, i))

decr.sort(reverse=True)
G = [[] for _ in range(N)]

for d, i, j in decr:
    if not bfs(i, j):
        ans -= d
        G[j].append(i)

print(ans)