import sys
input = sys.stdin.buffer.readline
sys.setrecursionlimit(10 ** 7)


class SCC_graph(object):
    def __init__(self, n):
        """n:ノード数"""
        self.n = n
        self.edges = []

    def add_edge(self, frm, to):
        """frm -> toへ有効辺を張る"""
        self.edges.append((frm, to))

    def __csr(self):
        self.start = [0] * (self.n + 1)
        self.elist = [0] * len(self.edges)
        for frm, to in self.edges:
            self.start[frm + 1] += 1
        for i in range(1, self.n + 1):
            self.start[i] += self.start[i - 1]
        cnt = self.start[:]
        for frm, to in self.edges:
            self.elist[cnt[frm]] = to
            cnt[frm] += 1

    def __dfs(self, v):
        self.low[v] = self.now_ord
        self.order[v] = self.now_ord
        self.now_ord += 1
        self.visited.append(v)
        for i in range(self.start[v], self.start[v + 1]):
            to = self.elist[i]
            if self.order[to] == -1:
                self.__dfs(to)
                self.low[v] = min(self.low[v], self.low[to])
            else:
                self.low[v] = min(self.low[v], self.order[to])
        if self.low[v] == self.order[v]:
            while self.visited:
                u = self.visited.pop()
                self.order[u] = self.n
                self.ids[u] = self.group_num
                if u == v:
                    break
            self.group_num += 1

    def _make_scc_ids(self):
        self.__csr()
        self.now_ord = 0
        self.group_num = 0
        self.visited = []
        self.low = [0] * self.n
        self.ids = [0] * self.n
        self.order = [-1] * self.n
        for i in range(self.n):
            if self.order[i] == -1:
                self.__dfs(i)
        for i in range(self.n):
            self.ids[i] = self.group_num - 1 - self.ids[i]

    def scc(self, make_adj=False):
        """
        強連結成分のリストをトポロジカル順に返す
        """
        self._make_scc_ids()
        groups = [[] for _ in range(self.group_num)]
        for i in range(self.n):
            groups[self.ids[i]].append(i)
        if not make_adj:
            return groups
        adj = [set() for _ in range(self.group_num)]
        for a, b in self.edges:
            a = self.ids[a]
            b = self.ids[b]
            if a == b:
                continue
            adj[a].add(b)
        return groups, adj


N = int(input())
dif = [0] * N
G = SCC_graph(N)
for i in range(N):
    dif[i], s = map(int, input().split())
    if i != s - 1:
        G.add_edge(s - 1, i)

group, adj = G.scc(True)
label = G.ids
half = [0]*len(group)
ans = 0

for g in group:
    d = [dif[i] for i in g]
    ans += sum(d)

    i = label[g[0]]
    if not half[i]:
        ans += min(d)
    for to in adj[i]:
        half[to] = 1

print("{:.1f}".format(ans*0.5))