class SCC():

    #index :0-index or 1-index
    #逆グラフは自動で作る
    def __init__(self,G,index = 1):
        self.G = G
        self.N = len(G)
        self.index = index
        self.rG = self.make_reverse()
        self.order = []
        self.parent = [0] * self.N  #各頂点の属する強連結成分の番号

        self.num = 0     #強連結成分の数
        
        self.build()
        self.sG,self.top,self.Component = self.make_sG()
        #sG 縮約後のグラフ。setでつくってある
        #top sGをトポロジカルソートしたもの
        #Component 各強連結成分に属する頂点をまとめた配列
        

    #逆グラフを作る
    def make_reverse(self):
        G = self.G
        rG = [[] for _ in range(self.N)]
        for v in range(self.index,self.N):
            for u in G[v]:
                rG[u].append(v)
        return rG
    def build(self):
        G = self.G
        rG = self.rG
        parent = self.parent
        order = self.order

        memo = [0] * self.N
        stack = []
        label = 0
        for v in range(self.index,self.N):
            if memo[v] == 1:continue
            stack.append(~v)
            stack.append(v)
            memo[v] = 1
            while stack:
                now = stack.pop()
                if now >= 0:
                    for u in G[now]:
                        if memo[u] == 0:
                            memo[u] = 1
                            stack.append(~u)
                            stack.append(u)
                else:
                    order.append(~now)
        memo = [0] * self.N
        for v in reversed(order):
            if memo[v] == 1:continue
            stack.append(v)
            memo[v] = 1
            parent[v] = label
            while stack:
                now = stack.pop()
                for u in rG[now]:
                    if memo[u] == 1:
                        continue
                    memo[u] = 1
                    stack.append(u)
                    parent[u] = label
            label += 1
        self.num = label
    #縮約後のグラフを作る
    #トポロジカルソートもする
    def make_sG(self):
        sG = [set() for _ in range(self.num)]
        Component = [[] for _ in range(self.num)]
        sGnum = [0] * self.num
        G = self.G
        parent = self.parent
        for v in range(self.index,self.N):
            Component[parent[v]].append(v)
            for u in G[v]:
                if parent[u] == parent[v]:continue
                if parent[u] not in sG[parent[v]]:
                    sG[parent[v]].add(parent[u])
                    sGnum[parent[u]] += 1
        top = []
        stack = []
        for i in range(self.num):
            if sGnum[i] == 0:
                stack.append(i)
        while stack:
            now = stack.pop()
            top.append(now)
            for v in sG[now]:
                sGnum[v] -= 1
                if sGnum[v] == 0:
                    stack.append(v)
        return sG,top,Component
        
N = int(input())
L = [0] * N
G = [[] for _ in range(N)]
for i in range(N):
    L[i],s = map(int,input().split())
    s -= 1
    if i != s:
        G[s].append(i)
scc = SCC(G,0)
Comp = scc.Component
top = scc.top
sG = scc.sG
ans = 0
memo = [0] * scc.num
for v in top:
    if memo[v] == 1:
        for u in Comp[v]:
            ans += L[u] / 2
    else:
        _min = 10 ** 9
        index = -1
        for u in Comp[v]:
            if L[u] < _min:
                _min = L[u]
                index = u
        ans += _min
        for u in Comp[v]:
            if u != index:
                ans += L[u] / 2
    for u in sG[v]:
        memo[u] = 1
print(ans/1)