class SCC: def __init__(self, n): self.n = n self.graph = [[] for i in range(n)] self.graph_rev = [[] for i in range(n)] self.used = [False]*n def add_edge(self, fr, to): if fr == to: return self.graph[fr].append(to) self.graph_rev[to].append(fr) def dfs(self, node, graph): self.used[node] = True for nex in graph[node]: if self.used[nex]: continue self.dfs(nex,graph) self.order.append(node) def first_dfs(self): self.used = [False]*self.n self.order = [] for i in range(self.n): if self.used[i]: continue self.dfs(i,self.graph) def second_dfs(self): self.used = [False]*self.n self.ans = [] for node in reversed(self.order): if self.used[node]: continue self.used[node] = True self.order = [] self.dfs(node, self.graph_rev) #self.order.reverse() self.ans.append(self.order) def scc(self): self.first_dfs() self.second_dfs() return self.ans N = int(input()) scc = SCC(N) L = [] for i in range(N): l,S = map(int,input().split()) scc.add_edge(S-1,i) L.append(l) ans = scc.scc() ans1 = 0 for i in ans: tmp = 0 m= 10 ** 5 for j in i: tmp += L[j] m = min(L[j],m) ans1 = max((tmp+m)/2,ans1) print(ans1)