# Input
N = int(input())
L = [ None ] * N
S = [ None ] * N
for i in range(N):
	L[i], S[i] = map(int, input().split())
	S[i] -= 1

# Calculation
answer = sum(L)
visited = [ -1 ] * N
for i in range(N):
	if visited[i] != -1:
		continue
	pos = i
	while visited[pos] == -1:
		visited[pos] = i
		pos = S[pos]
	if visited[pos] == i:
		min_increase = L[pos]
		tpos = S[pos]
		while tpos != pos:
			min_increase = min(min_increase, L[tpos])
			tpos = S[tpos]
		answer += min_increase

# Output
print(f'{answer / 2:.1f}')