import sys
input=lambda:sys.stdin.readline().rstrip()
class unionFind:
    def __init__(self,N):
        self.N=N
        self.parent=[-1 for i in range(N)]
        self.size=[1 for i in range(N)]
    def find(self,x):
        path=[x]
        while self.parent[path[-1]]!=-1:
            path.append(self.parent[path[-1]])
        for i in path[:-1]:
            self.parent[i]=path[-1]
        return path[-1]
    def unite(self,x,y):
        roots=sorted([self.find(x),self.find(y)],key=lambda _:self.parent[_])
        if roots[0]!=roots[1]:
            self.parent[roots[0]]=roots[1]
            self.size[roots[1]]+=self.size[roots[0]]
N=int(input())
diff=[0 for i in range(N)]
nex=[0 for i in range(N)]
prev=[0 for i in range(N)]
rem=[1 for i in range(N)]
ans=0
m={}
u=unionFind(N)
for i in range(N):
	diff[i],prev[i]=map(int,input().split())
	prev[i]-=1
	nex[prev[i]]+=1
	u.unite(i,prev[i])
erase=[]
for i in range(N):
	if nex[i]==0:
		erase.append(i)
while len(erase):
	temp=erase[-1]
	erase.pop()
	if rem[temp]==0:
		continue
	ans+=diff[temp]/2
	nex[prev[temp]]-=1
	rem[temp]=0
	if nex[prev[temp]]==0:
		erase.append(prev[temp])
for i in range(N):
	if rem[i]:
		ans+=diff[i]/2
		if not u.find(i) in m:
			m[u.find(i)]=float('inf')
		m[u.find(i)]=min(m[u.find(i)],diff[i])
for i in m:
	ans+=m[i]/2
print('{:.1f}'.format(ans))