import sequtils,strutils,strscans,algorithm,math
type
    unionfindtree[I : static[int]] = array[I,int]

proc Init(U : var unionfindtree)=
    for j in 0..<U.high:
        U[j] = j
    
proc find(U : unionfindtree; a : int, b :int): bool=
    var
        s = a
        t = b
    while s != U[s]:
        s = U[s]
    while t != U[t]:
        t = U[t]
    return s == t
    
proc union(U : var  unionfindtree; a : int ; b : int)=
    if U.find(a,b):
        return
    var
        s = a
        t = b
        t2 : int
    while s != U[s]:
        s = U[s]
    while t != U[t]:
        t2 = U[t]
        U[t] = s
        t = t2
    U[t] = s
proc root(U : unionfindtree, a :int):int=
    var
        s = a
    while s != U[s]:
        s = U[s]
    return s

type
    item = tuple[id : int,level : float,S : int]
var
    N = stdin.readline.parseInt
    oya = newSeqWith(N,-1)
    Items = newSeq[item](0)
    L : float
    S : int
    ans = 0.0
    p : item
    line : string
    s : set[int8]
    uft : unionfindtree[101]
    
uft.Init
for i in 0..<N:
    line = stdin.readline
    discard scanf(line,"$f $i",L,S)
    p = (i,L,S - 1)
    uft.union(S - 1,i)
    Items.add(p)
for i in 0..<N:
    s.incl(uft.root(i).int8)

for i in s:
    var j = i.int
    var minoya = 100000.0
    while oya[j] == -1:
        oya[j] = i.int
        minoya = min(minoya,Items[j].level)
        j = Items[j].S
    ans += minoya / 2

for t in Items:
    ans += t.level / 2

echo round(ans,1)