from collections import defaultdict
import sys
sys.setrecursionlimit(10 ** 6)


def scc(n, adj) -> list:
    def dfs(v, used, acc):
        used[v] = True
        for to in adj[v]:
            if used[to]: continue
            dfs(to, used, acc)
        acc.append(v)

    def r_dfs(v, used, g):
        used[v] = True
        g.append(v)
        for to in r_adj[v]:
            if used[to]: continue
            r_dfs(to, used, g)

    r_adj = defaultdict(list)  # 逆辺
    for k, vs in adj.items():
        for v in vs:
            r_adj[v].append(k)

    acc = []
    used = [False] * n
    for i in range(n):
        if used[i]: continue
        dfs(i, used, acc)

    cc = []
    used = [False] * n
    for node in reversed(acc):
        if used[node]: continue
        g = []
        r_dfs(node, used, g)
        cc.append(g)
    return cc


N = int(input())
adj = defaultdict(list)
ls =[]
for i in range(N):
    L, S = map(int, input().split())
    S -= 1
    # S がクリア済みだと i の難易度が半分になる
    adj[S].append(i)
    ls.append(L)

cc = scc(N, adj)
ans = 0
indegs = [0] * N
for c in cc:
    t = [ls[p] for p in c]
    if any(indegs[p] > 0 for p in c):
        ans += sum(t) / 2
    else:
        t.sort()
        ans += t[0]
        ans += sum(t[1:]) / 2

    for p in c:
        for to in adj[p]:
            indegs[to] += 1

print(ans)