from functools import reduce

n = int(input())
l, s = [], []
g = [[] for _ in range(n)]
for i in range(n):
    li, si = map(int, input().split())
    si -= 1
    l.append(li)
    s.append(si)
    g[si].append(i)

ans = reduce(lambda x, y: x + y, l) / 2.0
used = [False] * n
vs = []

def dfs(v):
    used[v] = True
    for i in g[v]:
        if not used[i]:
            dfs(i)
    vs.append(v)

Min = 0

def rdfs(v):
    global Min
    used[v] = True
    Min = min(Min, l[v])
    if not used[s[v]]:
        rdfs(s[v])

for i in range(n):
    if not used[i]:
        dfs(i)

used = [False] * n
used2 = [False] * n

def dfs2(v):
    used2[v] = True
    for i in g[v]:
        if not used2[i]:
            dfs2(i)

for i in reversed(range(len(vs))):
    if used[vs[i]] or used2[vs[i]]:
        continue
    Min = 100
    rdfs(vs[i])
    ans += Min / 2.0
    dfs2(vs[i])

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