N=int(raw_input())
outdeg = [0 for i in range(N+1)]
rev = [[] for i in range(N+1)]
LS=[(0,0) for i in range(N+1)]
for i in range(N):
   L,S=map(int,raw_input().split())
   LS[i+1]=(L,S)
   outdeg[S] += 1

num = 0
ans = 0.0
while num < N:
   ok = False
   m = 1000
   mi = -1
   for i in range(1,N+1):
      if outdeg[i] == 0:
         #print i,"half",LS[i][0]
         ans += LS[i][0]/2.0
         outdeg[i] = -1
         ok=True
         num+=1
         outdeg[LS[i][1]]-=1
      if outdeg[i] > 0:
         m = min(LS[i][0],m)
         if m == LS[i][0]:
            mi = i
   if ok == False:
      num+=1
      #print mi,"full",LS[mi][0]
      ans += LS[mi][0]
      outdeg[mi]=-1
      outdeg[LS[mi][1]]-=1
print ans