import pypyjit
pypyjit.set_param('max_unroll_recursion=-1')
from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
input = sys.stdin.readline

N = int(input())
e = [[] for _ in range(N)]
re = [[] for _ in range(N)]
L = []
for i in range(N):
    l,s = map(int,input().split())
    L.append(2*l)
    s -= 1
    if i==s:
        continue
    e[s].append(i)
    re[i].append(s)

# 強連結成分分解(SCC): グラフGに対するSCCを行う
# 入力: <N>: 頂点サイズ, <G>: 順方向の有向グラフ, <RG>: 逆方向の有向グラフ
# 出力: (<ラベル数>, <各頂点のラベル番号>)
def scc(N, G, RG):
    order = []
    used = [0]*N
    group = [None]*N
    def dfs(s):
        used[s] = 1
        for t in G[s]:
            if not used[t]:
                dfs(t)
        order.append(s)
    def rdfs(s, col):
        group[s] = col
        used[s] = 1
        for t in RG[s]:
            if not used[t]:
                rdfs(t, col)
    for i in range(N):
        if not used[i]:
            dfs(i)
    used = [0]*N
    label = 0
    for s in reversed(order):
        if not used[s]:
            rdfs(s, label)
            label += 1
    return label, group
    
    
n,group = scc(N,e,re)
G = [set() for _ in range(n)]
for i in range(N):
    gi = group[i]
    for j in e[i]:
        gj = group[j]
        if gj==gi:
            continue
        G[gj].add(gi)
X = [[] for _ in range(n)]
for i,g in enumerate(group):
    X[g].append(L[i])

ans = 0

for i in range(n):

    if len(G[i])==0:
        ans += sum(X[i])//2 + min(X[i])//2
    else:
        ans += sum(X[i])//2
print(ans*0.5)