class SCC(): #index :0-index or 1-index #逆グラフは自動で作る def __init__(self,G,index = 1): self.G = G self.N = len(G) self.index = index self.rG = self.make_reverse() self.order = [] self.parent = [0] * self.N #各頂点の属する強連結成分の番号 self.num = 0 #強連結成分の数 self.build() self.sG,self.top,self.Component = self.make_sG() #sG 縮約後のグラフ。setでつくってある #top sGをトポロジカルソートしたもの #Component 各強連結成分に属する頂点をまとめた配列 #逆グラフを作る def make_reverse(self): G = self.G rG = [[] for _ in range(self.N)] for v in range(self.index,self.N): for u in G[v]: rG[u].append(v) return rG def build(self): G = self.G rG = self.rG parent = self.parent order = self.order memo = [0] * self.N stack = [] label = 0 for v in range(self.index,self.N): if memo[v] == 1:continue stack.append(~v) stack.append(v) memo[v] = 1 while stack: now = stack.pop() if now >= 0: for u in G[now]: if memo[u] == 0: memo[u] = 1 stack.append(~u) stack.append(u) else: order.append(~now) memo = [0] * self.N for v in reversed(order): if memo[v] == 1:continue stack.append(v) memo[v] = 1 parent[v] = label while stack: now = stack.pop() for u in rG[now]: if memo[u] == 1: continue memo[u] = 1 stack.append(u) parent[u] = label label += 1 self.num = label #縮約後のグラフを作る #トポロジカルソートもする def make_sG(self): sG = [set() for _ in range(self.num)] Component = [[] for _ in range(self.num)] sGnum = [0] * self.num G = self.G parent = self.parent for v in range(self.index,self.N): Component[parent[v]].append(v) for u in G[v]: if parent[u] == parent[v]:continue if parent[u] not in sG[parent[v]]: sG[parent[v]].add(parent[u]) sGnum[parent[u]] += 1 top = [] stack = [] for i in range(self.num): if sGnum[i] == 0: stack.append(i) while stack: now = stack.pop() top.append(now) for v in sG[now]: sGnum[v] -= 1 if sGnum[v] == 0: stack.append(v) return sG,top,Component N = int(input()) L = [0] * N G = [[] for _ in range(N)] for i in range(N): L[i],s = map(int,input().split()) s -= 1 if i != s: G[s].append(i) scc = SCC(G,0) Comp = scc.Component top = scc.top sG = scc.sG ans = 0 memo = [0] * scc.num for v in top: if memo[v] == 1: for u in Comp[v]: ans += L[u] / 2 else: _min = 10 ** 9 index = -1 for u in Comp[v]: if L[u] < _min: _min = L[u] index = u ans += _min for u in Comp[v]: if u != index: ans += L[u] / 2 for u in sG[v]: memo[u] = 1 print(ans/1)